Correcting image gradation based on neighboring pixels

ABSTRACT

The invention provides an image processing apparatus and an image processing method. An area to which image data belongs is discriminated, and a correction coefficient to be used for correction of a pixel value of the image data is produced based on a result of the discrimination. Then, the pixel value of the image data is corrected with the correction coefficient. The relationship in magnitude among pixel values in the same area is maintained because the same coefficient is used, but pixel values which belong to different areas can be varied or even reversed. This allows the gradation of an entire image to be corrected while preventing partial deterioration of the contrast.

This is a divisional of U.S. patent application Ser. No. 10/802,600,filed Mar. 16, 2004 now U.S. Pat. No. 7,440,612, which is a divisionalapplication of U.S. patent application Ser. No. 09/434,565, filed Nov.12, 1999, now abandoned, the disclosures of which are incorporatedherein by reference. U.S. patent application Ser. No. 09/434,565 claimedpriority to Japanese Patent Application No. 10-323051, filed on Nov. 13,1998; Japanese Patent Application No. 10-327785, filed on Nov. 18, 1998;Japanese Application No. 10-328909, filed on Nov. 19, 1998; and JapaneseApplication No. 10-328937, filed on Nov. 19, 1998.

BACKGROUND OF THE INVENTION

This invention relates to an image processing apparatus and an imageprocessing method and can be applied to an image processing apparatussuch as, for example, a television receiver, a video tape recorder, atelevision camera and a printer.

Conventionally, an image processing apparatus such as a televisioncamera corrects the gradation of image data obtained from an imageinputting apparatus such as an image pickup apparatus and outputs theimage data of the corrected gradation.

FIG. 30 is a characteristic diagram showing an input/outputcharacteristic of a signal processing circuit adapted to such gradationcorrection processing. A signal processing circuit of the type describeddecreases the gain when the input level L increases higher than apredetermined reference level Lk. Consequently, a signal processingcircuit of the type described suppresses the signal level and outputsthe signal of the suppressed signal level when the input level is higherthan the reference level Lk. In this instance, the gradation iscorrected sacrificing the contract at a portion of an image having acomparatively high signal level.

In the characteristic diagram of FIG. 30, the axis of abscissarepresents the pixel value L which is the input level of image datawhile the axis of ordinate represents the pixel value T(L) which is theoutput level of the image data, and Lmax represents the maximum levelwhich can be taken by any pixel of the input/output images. In thefollowing description, a function indicative of an input/output functionas represented by the characteristic curve of FIG. 30 is referred to aslevel conversion function.

FIG. 31 is a characteristic diagram showing an input/outputcharacteristic of another signal processing circuit of a similar type.The signal processing circuit which uses the level conversion functionillustrated in FIG. 31 decreases the gain when the input level L islower than a first reference level Ls and when the input level L ishigher than a second reference level Lb. Consequently, the signalprocessing circuit corrects the gradation sacrificing the contrast wherethe signal level is comparatively low and where the signal level iscomparatively high with respect to an intermediate range of the inputsignal level.

On the other hand, in image processing and so forth wherein a computeris used, the gradation is corrected, for example, by histogramequalization.

The histogram equalization is a method of adaptively varying the levelconversion function in response to the frequency distribution of a pixelvalue of an input image, and corrects the gradation by reducing thegradation at a portion where the frequency distribution of the pixelvalue is low.

Referring to FIG. 32, in processing of the histogram equalization, acumulative frequency distribution C(L) by arithmetic processing of thefollowing expression (1) is detected based on a frequency distributionH(L) which is an aggregate of the pixel number with reference to thepixel value L of the input image:

$\begin{matrix}{{C(L)} = {\sum\limits_{k = 0}^{L}{H(k)}}} & (1)\end{matrix}$

In the processing of the histogram equalization, the cumulativefrequency distribution C(L) detected in this manner is normalized inaccordance with the following expression (2) to define a levelconversion function T(L), and the signal level of the input image iscorrected in accordance with the level conversion function T(L).T(L)=C(L)/Fmax×Lmax  (2)where Fmax is the final value of the cumulative frequency distributionC(L), and Lmax is the maximum value of the input/output levels

Such processing of correcting the gradation as described above isexecuted suitably in accordance with the necessity in order to suppressthe dynamic range or for some other object when image data aretransmitted over a transmission line, when image data are displayed on adisplay unit, when image data are stored into a storage device or in alike case.

In the correction processes of the gradation according to theconventional techniques described above, the entire gradation iscorrected sacrificing the contrast at some portion of the input image.This is because, with any of the techniques, the level is converted withan input/output function having a monotone increasing property in orderto prevent production of an unnatural image.

Accordingly, the conventional techniques have a problem in that an imageobtained by processing finally has a partially reduced contrast.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an image processingapparatus and an image processing method by which the gradation can becorrected while effectively preventing a finally obtained image fromsuffering from partial reduction in contrast.

In order to attain the object described above, according to the presentinvention, an area to which image data belongs is discriminated, forexample, with reference to a low frequency component of a pixel value,and the signal level of the image data is corrected based on a result ofthe discrimination to allow the gradation to be corrected whilepreventing partial deterioration in contrast effectively.

More particularly, in an image processing apparatus and an imageprocessing method according to an aspect of the present invention, anarea to which image data belongs is discriminated, and a correctioncoefficient to be used for correction of a pixel value of the image datais produced based on a result of the discrimination, and then the pixelvalue of the image data is corrected with the correction coefficient.

Where an area to which image data belongs is discriminated and acorrection coefficient to be used for correction of a pixel value of theimage data is produced based on a result of the discrimination and thenthe pixel value of the image data is corrected with the correctioncoefficient, the pixel values in the same area can be corrected with thesame coefficient to maintain the relationship in magnitude among thepixel values in the area, but the relationship in magnitude betweenpixel values which belong to different areas can be varied, for example,reversed. Consequently, the gradation of the entire image can becorrected while preventing partial deterioration of the contrast.

In an image processing apparatus and an image processing methodaccording to another aspect of the present invention, image data isdemultiplexed into brightness data and color data, and an area to whichthe brightness data belongs is discriminated, and then a correctioncoefficient to be used for correction of a pixel value of the brightnessdata is produced based on a result of the discrimination, whereafter thepixel values of the brightness data and the color data are correctedwith the correction coefficient.

Where image data is demultiplexed into brightness data and dolor dataand an area to which the brightness data belongs is discriminated andthen a correction coefficient to be used for correction of a pixel valueof the brightness data is produced based on a result of thediscrimination, whereafter the pixel values of the brightness data andthe color data are corrected with the correction coefficient, the pixelvalues in the same area can be corrected with the same coefficient tomaintain the relationship in magnitude among the pixel values in thearea, but the relationship in magnitude between pixel values whichbelong to different areas can be varied, for example, reversed.Consequently, the gradation of the entire image can be corrected whilepreventing partial deterioration of the contrast.

In an image processing apparatus and an image processing methodaccording to a further aspect of the present invention, a characteristicamount representative of a characteristic of a predetermined rangeneighboring to each pixel is successively detected, and an area to whichthe image data belongs is discriminated based on the characteristicamount, and then a correction coefficient is produced based on a resultof the discrimination and used for correction of the pixel value of theimage data.

Where an area to which image data belongs is discriminated and acorrection coefficient is produced based on a result of thediscrimination and used for correction of the pixel value of the imagedata, the pixel values in the same area can be corrected with the samecoefficient to maintain the relationship in magnitude among the pixelvalues in the area, but the relationship in magnitude between pixelvalues which belong to different areas can be varied, for example,reversed. Consequently, the gradation of the entire image can becorrected while preventing partial deterioration of the contrast. Inthis instance, where a characteristic amount representative of acharacteristic of a predetermined range neighboring to each pixel issuccessively detected and an area to which the image data belongs isdiscriminated based on the characteristic amount, for example, when anoutput signal of a solid-state image pickup device of the single platetype is processed directly as a processing object, the gradation can becorrected without any loss of color information superposed on thebrightness signal.

Further, in an image processing apparatus and an image processing methodaccording to a still further aspect of the present invention, an area towhich image data belongs is discriminated and a discrimination result isoutputted, and a correction coefficient to be used for correction of thepixel value of the image data is outputted based on the discriminationresult and the pixel value of the image data is corrected with thecorrection coefficient, the resolution of the correction coefficientbeing switched in response to the pixel value of the image data.

Where an area to which image data belongs is discriminated and adiscrimination result is outputted and then a correction coefficient tobe used for correction of the pixel value of the image data is outputtedbased on a result of the discrimination result and the pixel value ofthe image data is corrected with the correction coefficient, the pixelvalues in the same area can be corrected with the same coefficient tomaintain the relationship in magnitude among the pixel values in thearea, but the relationship in magnitude between pixel values whichbelong to different areas can be varied, for example, reversed.Consequently, the gradation of the entire image can be corrected whilepreventing partial deterioration of the contrast.

Upon such correction of the gradation, the contrast between differentareas depends upon the gradient of a level conversion function which isan image data input/output characteristic of correction means orcorrection processing, and as the spatial resolution of thediscrimination result increases, the influence of the level conversionfunction upon the result of the gradation correction increases.Accordingly, when the pixel value corresponds to a portion of the levelconversion function at which the gradient is small, if the resolution ofthe correction coefficient is increased, then even if, for example, thelevel conversion function does not keep a monotone increasing property,the influence of the level conversion function can be reduced to reducean unnatural variation of the contrast with respect to a neighboringarea. Consequently, also between adjacent areas, a natural contrast canbe assured.

The above and other objects, features and advantages of the presentinvention will become apparent from the following description and theappended claims, taken in conjunction with the accompanying drawings inwhich like parts or elements denoted by like reference symbols.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a television camera according to afirst embodiment of the present invention;

FIG. 2 is a plan view showing a color filter;

FIG. 3 is a signal waveform diagram illustrating a result of imagepickup when the color filter of FIG. 2 is used;

FIGS. 4(A) to 4(C) are characteristic diagrams illustrating processingof an image pickup result by the television camera of FIG. 1;

FIG. 5 is a schematic view illustrating an arrangement of pixel valuesby the television camera of FIG. 1;

FIG. 6 is a characteristic diagram illustrating a contrast correctioncoefficient g(i, j);

FIGS. 7(A) to 7(D) are signal waveform diagrams illustrating processingof a gradation correction circuit of the television camera of FIG. 1;

FIGS. 8(A) to 8(D) are signal waveform diagrams illustrating processingof the gradation correction circuit when the input level is differentfrom that in the case of FIGS. 7(A) to 7(D);

FIG. 9 is a block diagram showing a first modification to the televisioncamera of the first embodiment of the present invention;

FIG. 10 is a block diagram showing a second modification to thetelevision camera of the first embodiment of the present invention;

FIG. 11 is a block diagram showing a gradation correction circuitapplied to a third modification to the television camera of the firstembodiment of the present invention;

FIG. 12 is a signal waveform diagram illustrating operation of thegradation correction circuit of the FIG. 11 of the first embodiment ofthe present invention;

FIG. 13 is a block diagram showing a gradation correction circuitapplied to a fourth modification to the television camera of the firstembodiment of the present invention;

FIG. 14 is a block diagram showing a gradation correction circuitapplied to a fifth modification to the television camera of the firstembodiment of the present invention;

FIG. 15 is a block diagram showing a gradation correction circuitapplied to a sixth modification to the television camera of the firstembodiment of the present invention;

FIG. 16 is a block diagram showing a gradation correction circuitapplied to a seventh modification to the television camera of the firstembodiment of the present invention;

FIG. 17 is a block diagram showing a gradation correction circuitapplied to an eighth modification to the television camera of the firstembodiment of the present invention;

FIG. 18 is a block diagram showing a gradation correction circuitapplied to a television camera according to a second embodiment of thepresent invention;

FIG. 19 is a block diagram showing a gradation correction circuitapplied to a first modification to the television camera of the secondembodiment of the present invention;

FIG. 20 is a block diagram showing a gradation correction circuitapplied to a television camera according to a third embodiment of thepresent invention;

FIG. 21 is a block diagram showing a gradation correction circuitapplied to a first modification to the television camera of the thirdembodiment of the present invention;

FIG. 22 is a block diagram showing a gradation correction circuitapplied to a television camera according to a fourth embodiment of thepresent invention;

FIGS. 23(A) and 23(B) are signal waveform diagrams illustratingoperation of the gradation correction circuit of FIG. 22;

FIG. 24 is a block diagram showing a gradation correction circuitapplied to a first modification to the television camera of the fourthembodiment of the present invention;

FIG. 25 is a block diagram showing a gradation correction circuitapplied to a television camera according to a fifth embodiment of thepresent invention;

FIG. 26 is a block diagram showing a gradation correction circuitapplied to a first modification to the television camera of the fifthembodiment of the present invention;

FIG. 27 is a block diagram showing a gradation correction circuitapplied to a television camera according to a sixth embodiment of thepresent invention;

FIG. 28 is a block diagram showing a gradation correction circuitapplied to a first modification to the television camera of the sixthembodiment of the present invention;

FIG. 29 is a characteristic diagram illustrating a level conversionfunction applied to a gradation correction circuit applied to atelevision camera according to a different embodiment of the presentinvention;

FIG. 30 is a characteristic diagram illustrating a level conversionfunction applied to conventional suppression processing for a dynamicrange;

FIG. 31 is a characteristic diagram illustrating another levelconversion function applied to different conventional suppressionprocessing for a dynamic range; and

FIG. 32 is a characteristic diagram illustrating processing of histogramequalization.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the preferred embodiments of the present invention will bedescribed with reference to the accompanied drawings, as needed.

First Embodiment

Referring to FIG. 1, there is shown in block diagram a television cameraaccording to a first preferred embodiment of the present invention. Thetelevision camera is generally denoted at 1 and includes a CCDsolid-state image pickup device 2 driven by a timing generator (TG) 3.

FIG. 2 is a front elevational view showing, in an enlarged scale, animage pickup face of the CCD solid-state image pickup device 2.Referring to FIG. 2, a color filter of a diced arrangement ofcomplementary colors is disposed on the image pickup face of the CCDsolid-state image pickup device 2. More particularly, in the CCDsolid-state image pickup device 2, color filters of yellow (Ye) and cyan(Cy) are repeated in a unit of a pixel to form an odd-numbered-linewhile color filters of magenta (Mg) and green (G) are repeated in a unitof a pixel to form an even-numbered line.

Consequently, the CCD solid-state image pickup device 2 outputs, bymeans of a correlation double sampling circuit usually added to an imagepickup device of the type mentioned, an image pickup result whereinamplitude modulated color signals are successively superposedtime-divisionally on a brightness signal as seen in FIG. 3.

In order to output such an image pickup result as described above, theCCD solid-state image pickup device 2 obtains an image pickup result ina period of 1/60 second based on a charge accumulation time set by auser and outputs the image pickup result as an image pickup result VN bynormal exposure. Further, the CCD solid-state image pickup device 2obtains, within a vertical blanking period of the image pickup result VNby normal exposure, an image pickup result by another chargeaccumulation time which is shorter than the charge accumulation time bynormal exposure, and outputs the image pickup result as an image pickupresult VS by short time exposure.

Consequently, when the incoming light amount to the CCD solid-stateimage pickup device 2 is higher than a predetermined level, the imagepickup result VN by normal exposure which indicates that the outputlevel is saturated as seen in FIG. 4(A) and the image pickup result VSby short time exposure which indicates that the output level is notsaturated as seen in FIG. 4(B) because the charge accumulation time isshorter than that by normal exposure are output in a set from the CCDsolid-state image pickup device 2.

Referring back to FIG. 1, the television camera 1 further includes amemory 4N which receives, through a correlation double sampling circuit,a defect correction circuit, a matrix arithmetic circuit, an analog todigital conversion circuit and other necessary circuits all not shown,the image pickup result VN by normal exposure in the form of colorsignals of red, blue and green obtained by the processing of thecircuits mentioned, and temporarily holds and stores the image pickupresult VN by normal exposure.

The television camera 1 further includes a memory 4S which similarlyreceives, through the correlation double sampling circuit, defectcorrection circuit, matrix arithmetic circuit, analog to digitalconversion circuit and so forth all not shown, the image pickup resultVS by shorter time exposure, and temporarily holds and stores the imagepickup result VS by shorter time exposure.

An addition circuit 5 adds the image pickup result VN by normal exposurestored in the memory 4N and the image pickup result VS by shorter timeexposure stored in the memory 4S to obtain an image pickup result VTwhich has a broad dynamic range and has a sufficient pixel value, andoutputs the image pickup result VT. A level correction circuit 6corrects a pixel value of the image pickup result VS by shorter timeexposure outputted from the memory 4S so that the image pickup result VTfrom the addition circuit 5 may have a linearity sufficient forpractical use, and outputs the corrected pixel value.

Consequently, the television camera 1 produces an image pickup result VThaving a dynamic range significantly greater than that obtained by aconventional television camera as seen in FIG. 4(C).

A gradation correction circuit 8 corrects the pixel value of the imagepickup result VT to correct the gradation of the image pickup result VTand outputs the image pickup result VT of the corrected gradation. Asignal processing circuit 9 following the gradation correction circuit 8executes various signal processes necessary for the television camera toobtain an image pickup result and outputs the image pickup result to anexternal apparatus. Thereupon, the image pickup result is suppresseduniformly to suppress the dynamic range of the image pickup result so asto conform with the external apparatus, and the resulting image pickupresult is outputted to the external apparatus.

In the processing, the gradation correction circuit 8 executesarithmetic processing of the following expression (3) in advance toproduce a brightness signal Y from the image pickup result VT includingcolor signals R, G and B, corrects the gradations of the color signalsR, G and B with reference to the brightness signal Y, and outputsresulting color signals R, G and B.Y=0.3R+0.59G+0.11B  (3)

The gradation correction circuit 8 includes an area discriminationfilter 10 which discriminates an area to which image data of thebrightness signal Y belongs and outputs a result of the discrimination.Thereupon, the area discrimination filter 10 detects an averagebrightness level which is an average value of pixel values as acharacteristic amount which indicates a characteristic of apredetermined range neighboring to the image data, discriminates towhich average brightness level area the image data belongs, and outputsan average value which is the average brightness level as adiscrimination result.

In particular, the area discrimination filter 10 is a two-dimensionallow-pass filter and detects, from each of pixel values x(i, j) of thebrightness signal Y of the image pickup result VT successively inputtedthereto in the order of raster scanning, a low frequency component r(i,j) represented by the following expression (4), and outputs the lowfrequency component r(i, j) as a discrimination result.

$\begin{matrix}{{r( {i,j} )} = {\sum\limits_{{dj} = {- \frac{N}{2}}}^{\frac{N}{2}}{\sum\limits_{{di} = {- \frac{M}{2}}}^{\frac{M}{2}}\frac{x( {{i + {di}},{j + {dj}}} )}{M \times N}}}} & (4)\end{matrix}$where N and M are constants representative of the magnitude of theneighboring area for which an average value is calculated, and as seenfrom FIG. 5, in the television camera 1 of the present embodiment, inregard to the image pickup result VT inputted in the order of rasterscanning, the horizontal direction is indicated by the subscript i whilethe vertical direction is indicated by the subscript j. Consequently,the area discrimination filter 10 removes fine image structures fromwithin an image according to the image pickup result VT thereby toextract an area in which the pixel values are comparatively flat. It isto be noted that, since the area discrimination filter 10 is provided inorder to perform such processing as just described, preferably it has acomparatively narrow bandwidth.

Referring back again to FIG. 1, a coefficient calculation circuit 11 ofthe gradation correction circuit 8 produces a contrast correctioncoefficient g(i, j) using such a coefficient calculation function G as,for example, illustrated in FIG. 6 in response to the signal level ofthe low frequency component r(i, j). The coefficient calculationfunction G here is a function obtained by arithmetic processing of, forexample, the level conversion function T(L) described hereinabove withreference to FIG. 30 in accordance with the following expression (5):G(L)=T(L)/L  (5)

The coefficient calculation circuit 11 thus produces a contrastcorrection coefficient g(i, j) by arithmetic processing of the followingexpression (6):g(i,j)=G(r(i,j))  (6)

Thus, when the signal level of the low frequency component r(i, j) whichis an input level is in an area lower than a predetermined referencelevel Lk, the coefficient calculation circuit 11 outputs a contrastcorrection coefficient g(i, j) of a fixed value gmax higher than 1, butwhen the signal level of the f(i, j) is in another area equal to orhigher than the reference level Lk, the coefficient calculation circuit11 output a contrast correction coefficient g(i, j) which graduallydecreases to a value gmin in response to the signal level of the lowfrequency component r(i, j).

A multiplication circuit 12 of the gradation correction circuit 8multiplies the contrast correction coefficient g(i, j) produced in thismanner by the corresponding pixel value x(i, j) of the image pickupresult VT (in this instance, such multiplication is processing for eachcolor signal) to correct the signal level of the image pickup result VTwith the contrast correction coefficient g(i, j) and outputs the imagepickup result VT of the corrected signal level.

Operation of the First Embodiment

In the television camera 1 having the construction described above withreference to FIG. 1, the CCD solid-state image pickup device 2alternately outputs an image pickup result VN (FIG. 4(A)) by normalexposure according to a charge accumulation time set by a user andanother image pickup result VS (FIG. 4(B)) by shorter time exposureaccording to a shorter charge accumulation time. The image pickupresults VN and VS are stored into the memories 4N and 4S, respectively.In the television camera 1, the two image pickup results VN and VS arecomposed by the level correction circuit 6 and the addition circuit 5 sothat an image pickup result VT (FIG. 4(C)) of a dynamic range havingsignificantly greater than that by a conventional television camera isproduced.

A brightness signal Y is produced from the image pickup result VT, andan average value of pixel values which is a characteristic amountindicative of a characteristic of a predetermined neighboring range toeach input image data is detected by the area discrimination filter 10of the gradation correction circuit 8 thereby to produce adiscrimination result which indicates an area to which the input imagedata belongs. More particularly, the area discrimination filter 10detects a low frequency component r(i, j) which is an average value ofpixel values thereby to remove fine structures in the image and extractan area in which the pixel values are comparatively flat. The lowfrequency component r(i, j) is outputted as a discrimination result.

From the image pickup result VT, a contrast correction coefficient g(i,j) is produced in response to the signal level of the low frequencycomponent r(i, j) by the coefficient calculation circuit 11, and thepixel value is corrected with the contrast correction coefficient g(i,j) by the multiplication circuit 12 thereby to correct the pixel valuewith a gain according to each area with reference to the low frequency,component r(i, j) by the coefficient calculation circuit 11. The thuscorrected pixel value is outputted from the coefficient calculationcircuit 11.

Consequently, pixel values in an area of the image pickup result VT inwhich the signal levels of the low frequency components r(i, j) areequal are corrected with an equal gain, but pixel values in areas inwhich the signal levels of the low frequency components r(i, j) aredifferent can be made nearer to each other in accordance with a settingof the level conversion function T(L), or depending upon a case, therelationship between pixel values in regard to the magnitude canpossibly be reversed. By the processing, the contrast in each area canbe increased naturally with respect to the gradation of the entireimage, and the gradation of the entire image can be corrected whilepreventing a partial reduction of the contrast effectively.

In particular, when the pixel value x(i, j) of the image pickup resultVT is pulsated with a frequency higher than the cutoff frequency of thelow-pass filter 10 and the dc level of the pixel value x(i, j) risessuddenly (FIG. 7(B)) and besides the variation of the low frequencycomponent r(i, j) corresponding to the sudden variation of the dc levelcrosses an inflection point of a coefficient calculation function G(L)(FIG. 7(A)), where the conventional level conversion function describedhereinabove with reference to FIG. 30 is applied, the contrast issuppressed in a portion where the pixel value x(i, j) is high (FIG.7(C)).

However, with the television camera 1 of the present embodiment, beforeand after the signal level of the low frequency component r(i, j) risessuddenly, the pixel value x(i, j) is corrected with a gain correspondingto the signal level of the low frequency component r(i, j), and thesignal level is corrected in accordance with the setting of thecoefficient calculation function G(L). In this instance, where the pixelvalue x(i, j) is low, it is corrected with the gain gmax based on anaverage value level L2 between a peak value L3 and a bottom value L1.Consequently, in the low level area, a contrast substantially equal tothat obtained by the conventional method can be obtained (FIG. 7(D)).

In contrast, in a high level side area, the pixel value x(i, j) iscorrected with a gain g5 of an average value level L5 between a peakvalue L6 and a bottom value L4. In this instance, since the pixel valuesare corrected with a gain whose peak value L6 and bottom value L4 areuniform, the contrast between the peak value L6 and the bottom value L4is amplified with the gain g5.

Consequently, the television camera 1 of the present embodiment does notexhibit a great variation in gradation when an image is viewed as awhole, but can expand, when a pulsation is viewed microscopically, agreat pulsation by an image pickup result VT of an input image.

When the pixel value x(i, j) is pulsated similarly and the dc level ofit rises suddenly (FIG. 8(B)) and besides the pulsation of the pixelvalue x(i, j) is displaced to the high level side from an inflectionpoint of the coefficient calculation function G(L) (FIG. 8(A)), wherethe conventional level conversion function described hereinabove withreference to FIG. 30 is applied, the contrast is suppressed with regardto all pixel values x(i, j) (FIG. 8(C)).

Also in this instance, however, on the higher level side and the lowerlevel side, the pixel values are corrected with the gains g2 and g3corresponding to the average levels L2 and L5, respectively, andalthough the gradation when the image is viewed entirely does notexhibit a great variation, when a pulsation is viewed microscopically, agreat pulsation of the image pickup result VT of the input image can beexpanded.

Effects of the First Embodiment

With the television camera 1 having the construction described above,since an area to which each image data belongs is discriminated and acorrection coefficient to be used for correction of the pixel value ofthe image data is produced based on a result of the discrimination andthen the pixel value of the image data is corrected with the correctioncoefficient, while the relationship in magnitude among pixel values inthe same area is maintained because the same coefficient is used, pixelvalues which belong to different areas can be made nearer to each otherin accordance with the necessity, and in an extreme case, therelationship can be reversed. Consequently, the contrast in each areacan be expanded within a predetermined level range, and the gradation ofthe entire image can be corrected while preventing a partial reductionof the contrast.

Further, where a low frequency component obtained using low-pass filtersis used as a characteristic amount and the pixel value is corrected withreference to the low frequency component, the gradation of the entireimage can be corrected while preventing a partial reduction of thecontrast with a simple construction.

First Modification to the First Embodiment

A first modification to the television camera 1 of the first embodimentis shown in FIG. 9. The modified television camera 1 is different fromthe television camera 1 of the first embodiment described hereinabovewith reference to FIG. 1 in that it includes a gradation correctioncircuit 8A in place of the gradation correction circuit 8.

The gradation correction circuit 8A includes a pair of arithmeticcircuits 13A and 13B, an area discrimination filter 10, a coefficientcalculation circuit 11, and a pair of multiplication circuits 12C and12Y. The area discrimination filter 10 and coefficient calculationcircuit 11 are similar to those of the television camera 1 describedhereinabove with reference to FIG. 1, and overlapping description ofthem is omitted here to avoid redundancy.

The arithmetic circuit 13A receives an image pickup result VT (x(i, j))as an input thereto from the addition circuit 5 and demultiplexes theimage pickup result VT (x(i, j)) into brightness data and color data. Inparticular, the arithmetic circuit 13A is formed from one-dimensionallow-pass filters and executes arithmetic processing of the followingexpressions (7) to produce brightness data y(i, j) and color data c(i,j):y(i,j)=LPFy(x(i,y))c(i,j)=LPFc(vi×x(i,j))vi=1 . . . i=evenvi=−1 . . . i=odd  (7)where LPFy and LPFc represent characteristics of the one dimensionalfilters. Also in this instance, in regard to the image pickup result VTinputted in the order of raster scanning, the horizontal direction isindicated by the subscript i while the vertical direction is indicatedby the subscript j as seen in FIG. 5.

The area discrimination filter 10 discriminates an area to which eachbrightness data y(i, j) demultiplexed in this manner belongs, andoutputs a result of the discrimination. In this instance, the areadiscrimination filter 10 detects an average brightness level which is anaverage value of brightness data y(i, j) as a characteristic amountrepresentative of a predetermined range neighboring to the brightnessdata y(i, j) and outputs the average brightness level as adiscrimination result.

In particular, the area discrimination filter 10 is a two-dimensionallow-pass filter and detects, for each brightness data y(i, j) ofbrightness data successively inputted thereto in the order of rasterscanning, a low frequency component r(i, j) represented by the followingexpression (8) and outputs the low frequency component r(i, j) as adiscrimination result.

$\begin{matrix}{{r( {i,j} )} = {\sum\limits_{{dj} = {- \frac{N}{2}}}^{\frac{N}{2}}{\sum\limits_{{di} = {- \frac{M}{2}}}^{\frac{M}{2}}\frac{x( {{i + {di}},{j + {dj}}} )}{M \times N}}}} & (8)\end{matrix}$where N and M are constants representative of the magnitude of theneighboring area for calculation of an average value. Thus, the areadiscrimination filter 10 removes fine structures from within the imagebased the image pickup result VT to extract an area in which the pixelvalues are comparatively flat. It is to be noted that, since the areadiscrimination filter 10 is provided in order to perform such processingas just described, preferably it has a comparatively narrow bandwidth.

In addition, the coefficient calculation 11 is similar to that of thetelevision camera 1 described above with reference to FIG. 1.

The multiplication circuit 12Y multiplies the contrast correctioncoefficient g(i, j) produced in this manner by the correspondingbrightness data y(i, j) to correct the signal level of the image pickupresult VT based on the brightness data with the contrast correctioncoefficient g(i, j) and outputs the image pickup result VT of thecorrected signal level.

The multiplication circuit 12C similarly multiplies the contrastcorrection coefficient g(i, j) by the corresponding color data c(i, j)to correct the signal level of the image pickup result VT based on thecolor data with the contrast correction coefficient g(i, j) and outputsthe image pickup result VT of the corrected signal level.

The arithmetic circuit 13B executes arithmetic processing of thefollowing expression (9) to convert brightness data y′(i, j) and colordata c′(i, j), whose gradations have been corrected by such signal levelcorrection as described above, into original image data x′(i, j) andoutputs the original image data x′(i, j).x′(i,j)=y′(i,j)+Vic′(i,j)  (9)

In the television camera 1 having the construction described above, theimage pickup result VT is demultiplexed into brightness data y(i, j) andcolor data c(i, j) by the arithmetic circuit 13A of the gradationcorrection circuit 8A. Further, a characteristic amount indicative of acharacteristic of a predetermined range neighboring to each input imagedata is detected thereby to discriminate to which average brightnesslevel area the brightness data belongs. More particularly, the areadiscrimination filter 10 detects a low frequency component r(i, j) whichis an average brightness level of each brightness data y(i, j) therebyto remove fine structures in the image and extract areas in which thepixel values are comparatively flat. The area discrimination filter 10outputs the low frequency component r(i, j) as a discrimination result.

From the image pickup result VT, a contrast correction coefficient g(i,j) is produced in response to the signal level of the low frequencycomponent r(i, j) by the succeeding coefficient calculation circuit 11,and the pixel values of the brightness data y(i, j) and the color datac(i, j) are corrected with the contrast correction coefficient g(i, j)by the multiplication circuit 12Y and multiplication circuit 12C,respectively. Further, the image pickup result VT is returned into animage pickup result VT of the original form by the following arithmeticcircuit 13B. Consequently, the pixel values corrected with gainsaccording to the individual areas with reference to the low frequencycomponent r(i, j) are outputted from the arithmetic circuit 13B.

With the television camera 1 having the construction described above,since an area to which each input image data belongs is discriminatedand a correction coefficient is produced based on a result of thediscrimination and then an image pickup result is corrected inaccordance with the correction coefficient, while the relationship inmagnitude among pixel values in the same area is maintained because thesame coefficient is used, pixel values which belong to different areascan be made nearer to each other in accordance with the necessity, andin an extreme case, the relationship can be reversed. Consequently, thecontrast in each area can be expanded within a predetermined levelrange, and the gradation of the entire image can be corrected whilepreventing a partial reduction of the contrast.

In this instance, if, after an image pickup result is demultiplexed intobrightness data and color data, a correction coefficient is producedbased on a discrimination result of an area to which the brightness databelongs and the brightness data and the color data are corrected withthe correction coefficient to correct the gradation of the image pickupresult, the gradation can be corrected without any unfamiliar feelingwhile preventing occurrence of color noise effectively.

Further, where a low frequency component obtained using low-pass filtersis used as a characteristic amount and the pixel value is corrected withreference to the low frequency component, the gradation of the entireimage can be corrected while preventing a partial reduction of thecontrast with a simple construction.

Second Modification to the First Embodiment

A second modification to the television camera 1 of the first embodimentis shown in FIG. 10. The modified television camera 1 is different fromthe television camera 1 of the first embodiment described hereinabovewith reference to FIG. 1 in that it includes a gradation correctioncircuit 8C in place of the gradation correction circuit 8.

The gradation correction circuit 8C includes an area discriminationfilter 10, a coefficient calculation circuit 11, a multiplicationcircuit 12, and a characteristic amount filter 16. The areadiscrimination filter 10, coefficient calculation circuit 11 andmultiplication circuit 12 are similar to those of the gradationcorrection circuit 8 described above with reference to FIG. 1, andoverlapping description of them is omitted here to avoid redundancy.

The characteristic amount filter 16 of the gradation correction circuit8C in FIG. 10 detects a characteristic amount of each pixel value x(i,j) from an image pickup result VT inputted thereto from the additioncircuit 5, and outputs the detected characteristic amount. Thecharacteristic amount here indicates, for each of pixel values x(i, j)of the image pickup result VT successively inputted to thecharacteristic amount filter 16, a characteristic of a predeterminedrange around a pixel having the pixel value x(i, j). In the modifiedtelevision camera 1, the characteristic amount filter 16 is formed froma two-dimensional maximum value filter, and detects, for each of thepixel values x(i, j) of the image pickup result VT successively inputtedthereto, a maximum value of pixel values within the predetermined rangeneighboring to the pixel of the pixel value x(i, j) and outputs themaximum value xmax(i, j) as a characteristic amount of the pixel valuex(i, j).

In particular, the characteristic amount filter 16 processes pixelvalues x(i, j) successively inputted thereto by arithmetic processing ofthe following expressions (10) and successively outputs maximum valuesxmax(i, j) obtained by the arithmetic processing.xmax(i,j)=max(x(i+di,j+dj))−M/2≦di≦M/2−N/2≦dj≦N/2  (10)where max is a function for calculating a maximum value of x whichsatisfies a predetermined condition. The predetermined condition here isa range of M×N pixels around the pixel of the pixel value x(i, j). It isto be noted that, in regard to the image pickup result VT inputted inthe order of raster scanning, the horizontal direction is indicated bythe subscript i while the vertical direction is indicated by thesubscript j as seen in FIG. 5.

The area discrimination filter 10 discriminates an area to which theinput image data belongs with reference to the maximum value xmax(i, j)detected in this manner, and outputs a result of the discrimination. Inthis instance, the area discrimination filter 10 detects an averagevalue of the maximum values xmax(i, j), discriminates to which averagebrightness level area the input image data belongs, and outputs theaverage value as an identification signal.

In particular, the area discrimination filter 10 is a two-dimensionallow-pass filter, and detects, for each of maximum values xmax(i, j)successively inputted thereto in the order of raster scanning, a lowfrequency component r(i, j) represented by the following expression (11)and outputs such low frequency component r(i, j) as identificationsignal of each area.

$\begin{matrix}{{r( {i,j} )} = {\sum\limits_{{dj} = {- \frac{N}{2}}}^{\frac{N}{2}}{\sum\limits_{{di} = {- \frac{M}{2}}}^{\frac{M}{2}}\frac{x\;{\max( {{i + {di}},{j + {dj}}} )}}{M \times N}}}} & (11)\end{matrix}$where N and M are constants representing the magnitude of theneighboring area for calculation of an average value, but have valuesindependent of the values N and M appearing in the expression (3) above.Thus, the area discrimination filter 10 removes fine structures from theimage pickup result VT with reference to the maximum values xmax(i, j)to extract areas in which the pixel values are comparatively flat. It isto be noted that, since the area discrimination filter 10 is provided inorder to perform such processing as just described, preferably it has acomparatively narrow bandwidth.

From the image pickup result VT, a characteristic amount of each pixelvalue x(i, j) is detected by the characteristic amount filter 16 of thegradations correction circuit 8C. In other words, from the image pickupresult VT, the maximum values xmax(i, j) of pixel values withinpredetermined neighboring areas to the individual pixels are detected ascharacteristic amounts of the corresponding pixel values x(i, j).

From the image pickup result VT, an area to which each input image databelongs is discriminated based on the corresponding maximum valuexmax(i, j) detected in this manner by the area discrimination filter 10,and a result of the discrimination is outputted from the areadiscrimination filter 10. More specifically, the area discriminationfilter 10 detects low frequency components r(i, j) which are averagevalues of the maximum values xmax(i, j) and thereby eliminates finestructures in the image and extracts areas in which the pixel values arecomparatively flat. Further, the low frequency components r(i, j) areoutputted as identification signals of the individual areas.

In this instance, since, in the image pickup result VT obtained byfiltering with a color filter of a diced arrangement of complementarycolors as described above, an average of neighboring pixel valuesrepresents brightness information (FIG. 3) and a maximum value of thelow frequency component r(i, j) is detected as maximum value xmax(i, j)by the characteristic amount filter 16, the low frequency component r(i,j) varies in response to the amplitude of the color signal superposed onthe brightness signal.

Consequently, since the characteristic amount filter detects a maximumvalue xmax(i, j) of pixel values in a predetermined range neighboring toeach pixel as a characteristic amount and obtains a discriminationresult with reference to the characteristic amount, the gradation can becorrected while effectively preventing saturation of the pixel valuey(i, j), and consequently, the color can be regenerated correctly.

Third Modification to the First Embodiment

FIG. 11 shows a gradation correction circuit applied to a thirdmodification to the television camera 1 of the first embodiment shown inFIG. 1. Referring to FIG. 11, the gradation correction circuit isgenerally denoted at 18 and applied in place of the gradation correctioncircuit 8C of the modified television camera 1 described hereinabovewith reference to FIG. 10. The gradation correction circuit 18 includesa characteristic amount filter 22, an area discrimination filter 10, acoefficient calculation circuit 21, and a multiplication circuit 12. Thearea discrimination filter 10 and multiplication circuit 12 are similarto those described hereinabove with reference to FIG. 10 or 1, andoverlapping description of them is omitted here to avoid redundancy.

The characteristic amount filter 22 in the gradation correction circuit18 detects a characteristic amount of each pixel value x(i, j) of animage pickup result VT inputted thereto from the addition circuit 5(FIG. 10) and outputs the detected characteristic amount. Thecharacteristic amount filter 22 is a two-dimensional minimum valuefilter, and detects, for each one of pixel values x(i, j) of the imagepickup result VT successively inputted thereto, a minimum value xmin(i,j) of pixel values within a predetermined range around a pixel of thepixel value x(i, j) and outputs the minimum value xmin(i, j) as acharacteristic amount of the pixel value x(i, j).

In particular, the characteristic amount filter 22 processes the pixelvalues x(i, j) successively inputted thereto by arithmetic processing ofthe following expressions (12) and successively outputs minimum valuesxmin(i, j) obtained by the arithmetic processing.xmin(i,j)=min(x(i+di,j+dj))−M/2≦di≦M/2−N/2≦dj≦N/2  (12)where min is a function for calculating a minimum value of x whichsatisfies a predetermined condition.

The coefficient calculation circuit 21 produces a contrast correctioncoefficient g(i, j) in accordance with a coefficient calculationfunction G obtained by arithmetic processing of the level conversionfunction T(L) illustrated in FIG. 12 in accordance with the expression(5) in response to the signal level of the low frequency component r(i,j). The level conversion function T(L) here is a characteristic set sothat the signal level may be suppressed in an image portion in which thebrightness level is lower than the predetermined reference level Lk.

Where the correction coefficient g(i, j) is set in this manner, if thegain is controlled merely with the low frequency component of the pixelvalue x(i, j), then there is the possibility that, contrary to the caseof the second modification to the television camera 1 of the firstembodiment shown in FIG. 1, in an image portion in which the signallevel of the low frequency component r(i, j) is low, the pixel valuey(i, j) of the corrected gradation may be saturated, resulting in lossof color information. Therefore, correct color regeneration in such animage portion as described above is difficult.

However, where the gradation correction circuit 18 is employed, since aminimum value of a pixel value x(i, j) is detected as a characteristicamount xmin(i, j), in an image portion in which such saturation asdescribed above may possibly occur, the average brightness level r(i, j)as an identification result can be decreased to increase the gain upongradation correction, and consequently, saturation of the pixel valuey(i, j) can be prevented effectively as much and color regeneration canbe performed correctly.

Where the gradation correction circuit 18 is employed as shown in FIG.11, since a minimum value in a predetermined area is detected as acharacteristic amount and used for correction of the gradation, evenwhere the gradation is corrected so as to suppress the signal in animage area in which the brightness level is lower than the referencelevel Lk, similar effects to those achieved by the modified televisioncamera 1 of FIG. 10 can be achieved.

Fourth Modification to the First Embodiment

FIG. 13 shows a gradation correction circuit applied to a fourthmodification to the television camera 1 of the first embodiment shown inFIG. 1. Referring to FIG. 13, the gradation correction circuit isgenerally denoted at 18A and applied in place of the gradationcorrection circuit 8C of the modified television camera 1 describedhereinabove with reference to FIG. 10. The gradation correction circuit18A includes a characteristic amount filter 24, an area discriminationfilter 10, a coefficient calculation circuit 25, and a multiplicationcircuit 12. The area discrimination filter 10 and multiplication circuit12 are similar to those described hereinabove with reference to FIG. 10or 1, and overlapping description of them is omitted here to avoidredundancy.

The characteristic amount filter 24 in the gradation correction circuit18A detects and outputs a characteristic amount of each pixel value x(i,j) of an image pickup result VT. More particularly, the characteristicamount filter 24 includes a maximum value filter 24A which isconstructed similarly to the characteristic amount filter 16 describedhereinabove with reference to FIG. 10 and detects and outputs, for eachof pixel values x(i, j) of the image pickup result VT successivelyinputted thereto, a maximum value xmax(i, j) of pixel values within apredetermined range neighboring to the pixel having the pixel value x(i,j).

The characteristic amount filter 24 further includes a minimum valuefilter 24B which is constructed similarly to the characteristic amountfilter 22 described hereinabove with reference to FIG. 11 and detectsand outputs, for each of the pixel values x(i, j) of the image pickupresult VT successively inputted thereto, a minimum value xmin(i, j) ofpixel values in the predetermined range neighboring to the pixel havingthe pixel value x(i, j).

The characteristic amount filter 24 further includes a low-pass filter24C in the form of a two-dimensional low-pass filter and detects andoutputs, for each of the pixel values x(i, j) of the image pickup resultVT successively inputted thereto, an average value xave(1, j) of pixelvalues. It is to be noted that the low-pass filter 24C is set such thatthe constants M and N which define the magnitude of a neighboring arearepresented by the expression (11) given hereinabove have lower valuesthat those of the area discrimination filter 10 so that that thelow-pass filter 24C may have a pass-band width greater than that of thearea discrimination filter 10.

The characteristic amount filter 24 further includes a selector 24Dwhich compares the average value xave(i, j) outputted from the low-passfilter 24C with a predetermined reference value and selectively outputsthe maximum value xmax(i, j) outputted from the maximum value filter 24Aor the minimum value xmin(i, j) outputted from the minimum value filter24B based on a result of the comparison. In particular, when the averagevalue xave(i, j) is higher than the reference level, the selector 24Dselectively outputs the maximum value xmax(i, j) outputted from themaximum value filter 24A, but when the average value xave(i, j) is lowerthan the reference level, the selector 24D selectively outputs theminimum value xmin(i, j) outputted from the minimum value filter 24B.Consequently, the selector 24D composes the maximum value xmax(i, j) andthe minimum value xmin(i, j) to obtain a characteristic amount xmm(i, j)and outputs the characteristic amount xmm(i, j).

The coefficient calculation circuit 25 produces a contrast correctioncoefficient g(i, j) from a coefficient calculation function G obtainedby arithmetic processing of the level conversion function T(L)illustrated in FIG. 31 in accordance with the expression (5) givenhereinabove in response to the signal level of the low frequencycomponent r(i, j).

In this instance, when the contrast correction coefficient g(i, j) isset in this manner, if the gain is controlled merely with a lowfrequency component of the pixel value x(i, j), then there is thepossibility that color information may be lost in image portions inwhich the signal level of the low frequency component r(i, j) iscomparatively high and comparatively low, and correct color regenerationin such image portions is difficult.

Where the gradation correction circuit 18A is employed, however, since alow frequency component r(i, j) is produced from a characteristic amountxmm(i, j) obtained by composition by switching the maximum value xmax(i,j) and the minimum value xmin(i, j) of the pixel value x(i, j) with theaverage value xave(i, j) of the pixel value x(i, j) and the gain iscontrolled with the low frequency component r(i, j), the gain upongradation correction can be increased or decreased in an image portionin which such saturation as described above may possibly occur, andcorrect color regeneration can be achieved while effectively preventingsaturation of the pixel value y(i, j) effectively as much.

Where the gradation correction circuit 18A shown in FIG. 13 is employed,since a characteristic amount xmm(i, j) is produced by switching amaximum value xmax(i, j) and a minimum value xmin(i, j) of a pixel valuex(i, j) with an average value xave(i, j) of the pixel value x(i, j),even when the gradation is corrected so that the signal level may besuppressed in an image portion in which the brightness level is lowerthan a predetermined reference level Ls and another image portion inwhich the signal level is higher than another reference level Lb,similar effects to those achieved by the television camera 1 of FIG. 1can be achieved.

It is to be noted that, in the gradation correction circuit 18A shown inFIG. 13, a weighting addition circuit may be provided in place of theselector 24D.

In particular, the weighting addition circuit in this instance executesarithmetic processing of the following expressions (14) based on anaverage value xave(i, j) of pixel values x(i, j) outputted from thelow-pass filter 24C to produce a weighting coefficient a:

$\begin{matrix}\begin{matrix}\begin{matrix}{a = {{0.0\mspace{14mu}\ldots\mspace{14mu}{{xave}( {i,j} )}} < {THL}}} \\{{a = {{\{ {{{xave}( {i,j} )} - {THL}} \}/( {{THH} - {THL}} )}\mspace{14mu}\ldots}}\mspace{14mu}} \\{{THL} \leq {{xave}( {i,j} )} \leq {THH}} \\{a = {{1.0\mspace{14mu}\ldots\mspace{14mu}{{xave}( {i,j} )}} > {THH}}}\end{matrix} & \;\end{matrix} & (13)\end{matrix}$where THL and THH are constants for normalization.

Further, the weighting addition circuit executes arithmetic processingof the following expression (14) using the weighting coefficient aproduced in this manner thereby to compose the maximum value xmax(i, j)and the minimum value xmin(i, j) to produce a characteristic amountxmm(i, j).xmm(i,j)=a×xmax(i,j)+(1−a)×xmin(i,j)  (14)Consequently, the weighting addition circuit produces the characteristicamount xmm(i, j) by weighted averaging of the maximum value xmax(i, j)and the minimum value xmin(i, j) with reference to the average valuexave(i, j) of the pixel value x(i, j).

Where the weighting addition circuit is employed, since the maximumvalue xmax(i, j) and the minimum value xmin(i, j) can be composedsmoothly with reference to the average value xave(i, j) of the pixelvalue x(i, j) to produce the characteristic amount xmm(i, j), similaradvantages to those achieved by the television camera 1 of the forthmodification to the first embodiment can be achieved.

Fifth Modification to the First Embodiment

FIG. 14 shows a gradation correction circuit applied to a fifthmodification to the television camera 1 of the first embodiment shown inFIG. 1. Referring to FIG. 14, the gradation correction circuit isgenerally denoted at 18B and applied in place of the gradationcorrection circuit 8C of the modified television camera 1 describedhereinabove with reference to FIG. 10. The gradation correction circuit18B includes a characteristic amount filter 16, a quantization circuit43, an area discrimination filter 40, a lookup table (LUT) 44, and amultiplication circuit 12. The characteristic amount filter 16 andmultiplication circuit 12 are similar to those described hereinabovewith reference to FIG. 10, and overlapping description of them isomitted here to avoid redundancy.

The quantization circuit 43 re-quantizes a characteristic value xmax(i,j) to decrease the bit number of the characteristic amount xmax(i, j)and outputs the characteristic amount xmax(i, j) of the reduced bitnumber as a characteristic amount xmaxq(i, j). In particular, thequantization circuit 43 executes arithmetic processing of the followingexpression (15) with a quantization step Q set in advance for a pixelvalue x(i, j) to linearly quantize the characteristic amount xmax(i, j)to produce and output a characteristic amount xmaxq(i, j).xmaxq(i,j)=int{xmax(i,j)/Q+0.5}  (15)where int(a) is a function of discarding the fraction of a.

The area discrimination filter 40 is formed similarly to the areadiscrimination filter 10 described hereinabove in the secondmodification to the first embodiment with reference to FIG. 10 exceptthat it processes a signal of a different bit number.

The lookup table (LUT) 44 forms a coefficient calculation circuit andoutputs a correction coefficient g(i, j) using a low frequency componentr(i, j) outputted from the area discrimination filter 40 as an address.To this end, the lookup table 44 stores a correction coefficient LUT(i)given by the following expression (16) at an ith address thereof.LUT(i)=G(i×Q)  (16)

Where the gradation correction circuit 18B shown in FIG. 14 is employed,since a characteristic amount is quantized in advance, similar effectsto those achieved by the television camera 1 which employs the areadiscrimination filter 10 described hereinabove with reference to FIG. 10can be achieved by the television camera 1 having a more simplifiedconstruction. Further, since a lookup table is used to produce acorrection coefficient, the processing of the entire gradationcorrection circuit 18B can be simplified. Furthermore, since acharacteristic amount quantized in advance is used, the construction ofthe area discrimination filter can be simplified and the lookup tablecan be reduced in scale.

Sixth Modification to the First Embodiment

FIG. 15 shows a gradation correction circuit applied to a sixthmodification to the television camera 1 of the first embodiment shown inFIG. 1. Referring to FIG. 15, the gradation correction circuit isgenerally denoted at 18C and applied in place of the gradationcorrection circuit 18B described hereinabove with reference to FIG. 14.The gradation correction circuit 18C includes a characteristic amountfilter 16, a quantization circuit 43, an area discrimination filter 40,a lookup table 54, an interpolation circuit 55, and a multiplicationcircuit 12. Thus, the gradation correction circuit 18C is a modificationto and different from the gradation correction circuit 18B describedhereinabove with reference to FIG. 14 in that it includes the lookuptable 54 and the interpolation circuit 55 in place of the lookup table44. The characteristic amount filter 16, quantization circuit 43, areadiscrimination filter 40 and multiplication circuit 12 are similar tothose described hereinabove with reference to FIG. 14, and overlappingdescription of them is omitted here to avoid redundancy.

The lookup table 54 in the gradation correction circuit 18C has a numberof addresses smaller than the number of levels which can be assumed bythe output value r(i, j) of the area discrimination filter 40, and isaccessed with a value of the output value r(i, j) whose predeterminedlower bits are omitted. When the lookup table 54 is accessed in thismanner, it outputs two addresses addr0(i, j) and addr1(i, j) representedby the following expressions (17) and two correction coefficients g0(i,j) and g1(i, j).addr0(i,j)=int{r(i,j)/Rmax×R′max}addr1(i,j)=addr0(i,j)+1  (17)where Rmax is a maximum value which can be assumed by the output valuex(i, j) of the area discrimination filter 40, and R′max is a maximumvalue which can be assumed by the address of the lookup table 54.

It is to be noted that the lookup table 54 produces the address addr0(i,j) by omitting lower bits of the output value r(i, j) of the areadiscrimination filter 40 and produces the address addr1(i, j) by addinga bit of the logic 1 to the lowest bit of the address addr0(i, j).

The interpolation circuit 55 executes interpolation arithmeticprocessing in accordance with the following expressions (18) using theaddresses addr0(i, j) and addr1(i, j) and the correction coefficientsg0(i, j) and g1(i, j) inputted thereto from the lookup table 54 andoutputs a result of the interpolation as a correction coefficient g(i,j).

$\begin{matrix}{{g( {i,j} )} = {{{\{ {{r^{\prime}( {i,j} )} - {{addr}\; 0( {i,j} )}} \}/\{ {{{addr}\; 1( {i,j} )} - {{addr}\; 0\;( {i,j} )}} \}} \times \{ {{g\; 1( {i,j} )} - {g\; 0( {i,j} )}} \}} + {g\; 0( {i,j} )}}} & \; \\{{r^{\prime}( {i,j} )} = {{{r( {i,j} )}/R}\;\max \times R^{\prime}\max}} & (18)\end{matrix}$

Where the gradation correction circuit 18C shown in FIG. 15 is employed,since interpolation arithmetic processing is performed to produce acorrection coefficient, a correction coefficient whose value exhibits asmooth variation can be produced using a lookup table of a comparativelysmall scale, and the gradation can be corrected with a higher degree ofaccuracy as much.

Seventh Modification to the First Embodiment

FIG. 16 shows a gradation correction circuit applied to a seventhmodification to the television camera 1 of the first embodiment shown inFIG. 1. Referring to FIG. 16, the gradation correction circuit isgenerally denoted at 18D and applied in place of the gradationcorrection circuit 8C of the modified television camera 1 describedhereinabove with reference to FIG. 10. The gradation correction circuit18D includes a characteristic amount filter 16, an area discriminationfilter 60, a coefficient calculation circuit 11, and a multiplicationcircuit 12. The characteristic amount filter 16, coefficient calculationcircuit 11 and multiplication circuit 12 are similar to those describedhereinabove with reference to FIG. 10, and overlapping description ofthem is omitted here to avoid redundancy.

The area discrimination filter 60 in the gradation correction circuit18D includes a low-pass filter section 60A which discriminates an areato which input image data belongs with different resolutions to obtainand output identification signals r0(i, 1), r1(i, j), r2(i, j), . . .rN−1(i, j), and a signal composition section 60B for producing anidentification signal r(i, j) of a single composite signal based on theidentification signals r0(i, 1), r1(i, j), r2(i, j), . . . rN−1(i, j)according to the different resolutions.

The low-pass filter section 60A is formed from low-pass filters (LPF)F0, F1, F2, . . . , FN−1 having different pass-band widths. Acharacteristic amount xmax(i, j) from the characteristic amount filter16 is inputted to the low-pass filters F0, F1, F2, . . . , FN−1, andcorresponding frequency components are outputted as identificationsignals r0(i, j), r1(i, j), r2(i, j), . . . , rN−1(i, j) from thelow-pass filters F0, F1, F2, . . . , FN−1, respectively.

The signal composition section 60B includes multiplication circuits M0,M1, M2, . . . , MN−1 which receive and weight the identification signalsr0(i, j), r1(i, j), r2(i, j), . . . , rN−1(i, j), and an additioncircuit 66 which adds the weighted identification signals r0(i, j),r1(i, j), r2(i, j), . . . , rN−1(i, j) to produce a composite signal asan identification signal r(i, j). The identification signal r(i, j) thusobtained by the addition circuit 66 is outputted from the signalcomposition section 60B. It is to be noted that weighting coefficientsw0, w1, w2, . . . , wN−1 which are used by the multiplication circuitsM0, M1, M2, . . . , MN−1, respectively, are set in advance so that theymay satisfy the following relational expression (19):

$\begin{matrix}{{\sum\limits_{k = 0}^{N - 1}{wk}} = 1} & (19)\end{matrix}$

Consequently, in the gradation correction circuit 18D described above, aprofile provided by the image pickup result VT is not emphasizedabnormally according to the setting of the weighting coefficients w0,w1, w2, . . . , wN−1.

In particular, when the pixel value x(i, j) varies suddenly as seen inFIG. 23(A), the signal level of the low frequency component r(i, j)varies so that such a sudden variation of the pixel value may bemoderated. When the variation of the low frequency component r(i, j) ofthe pixel value x(i, j) is displaced to the higher level side withrespect to the inflection point of the characteristic describedhereinabove with reference to FIG. 6, if the correction coefficient g(i,j) is produced merely based on an output signal of a low-pass filter asin the television camera 1 of the second modification of FIG. 10, thenthe pixel value is amplified with an excessively high gain immediatelybefore the pixel value x(i, j) varies suddenly, but immediately afterthe pixel value x(i, j) varies suddenly, the pixel value is amplifiedwith an excessively low gain. Consequently, an output value y(i, j)(FIG. 23(B)) which provides an abnormally amplified profile is obtained.

In this instance, such abnormal emphasis of the profile as justdescribed can be reduced by correcting the pixel values with asubstantially uniform gain.

Consequently, where the gradation correction circuit 18D shown in FIG.16 is employed, since a correction coefficient is produced from aplurality of different low frequency components, abnormal emphasis of aprofile can be prevented effectively, and similar advantages to thoseachieved by the television camera 1 of the first embodiment and thesecond modification to it described hereinabove with reference to FIGS.1 and 10, respectively, can be achieved.

Eighth Modification to the First Embodiment

FIG. 17 shows a gradation correction circuit applied to an eighthmodification to the television camera 1 of the first embodiment shown inFIG. 1. Referring to FIG. 17, the gradation correction circuit isgenerally denoted at 18E and applied in place of the gradationcorrection circuit 8C of the modified television camera 1 describedhereinabove with reference to FIG. 10. The gradation correction circuit18E includes a characteristic amount filter 16, an area discriminationfilter 70, a coefficient calculation circuit 71, and a multiplicationcircuit 12. The characteristic amount filter 16 and multiplicationcircuit 12 are similar to those described hereinabove with reference toFIG. 10, and overlapping description of them is omitted here to avoidredundancy.

The area discrimination filter 70 in the gradation correction circuit18E discriminates an area to which input image data belongs withdifferent resolutions with reference to a characteristic amount xmax(i,j) and outputs discrimination results r0(i, j), r1(i, j), r2(i, j), . .. , rN−1(i, j) of the area. In particular, the area discriminationfilter 70 is formed from low-pass filters (LPF) F0, F1, F2, . . . , FN−1having different pass-band widths. A characteristic amount xmax(i, j)from the characteristic amount filter 16 is inputted to the low-passfilters F0, F1, F2, . . . , FN−1, and corresponding frequency componentsare outputted as identification signals r0(i, j), r1(i, j), r2(i, j), .. . , rN−1(i, j) from the low-pass filters F0, F1, F2, . . . , FN−1,respectively.

The coefficient calculation circuit 71 includes a coefficient productionsection 71A for producing, from the identification signals r0(i, j),r1(i, j), r2(i, j), . . . , rN−1(i, j), corresponding correctioncoefficients g0(i, j), g1(i, j), g2(i, j), . . . , gN−1(i, j), and acoefficient composition section 71B for composing the correctioncoefficients g0(i, j), g1(i, j), g2(i, j), . . . , gN−1(i, j) to producea single correction coefficient g(i, j).

The coefficient production section 71A includes coefficient calculationsections L0, L1, L2, . . . , LN−1 for producing, from the identificationsignals r0(i, j), r1(i, j), r2(i, j), . . . , rN−1(i, j), correspondingcorrection coefficients g0(i, j), g1(i, j), g2(i, j), . . . , gN−1(i, j)based on predetermined respective coefficient calculation functions Gk(k=0, 1, 2, . . . , N−1).

The coefficient composition section 71B includes multiplication circuitsM0, M1, M2, . . . , MN−1 which weight the correction coefficients g0(i,j), g1(i, j), g2(i, j), . . . , gN−1(i, j), and an addition circuit 76which adds results of the weighting by the multiplication circuits M0,M1, M2, . . . , MN−1 to produce and output a single correctioncoefficient g(i, j). It is to be noted that weighting coefficients w0,w1, w2, . . . , wN−1 used by the multiplication circuits M0, M1, M2, . .. , MN−1, respectively, are set in advance so that the relationalexpression (13) given hereinabove may be satisfied.

Where the gradation correction circuit 18E shown in FIG. 17 is employed,correction coefficients are produced from a plurality of different lowfrequency components and a signal correction coefficient is producedfrom the produced correction coefficients, and consequently, similaradvantages to those achieved by the gradation correction circuit 18Ddescribed hereinabove with reference to FIG. 16 can be achieved.

Second Embodiment

FIG. 18 is a block diagram showing a gradation correction circuitapplied to a television camera according to a second preferredembodiment of the present invention. The gradation correction circuit isgenerally denoted at 28 and is adapted in place of the gradationcorrection circuit 8 described hereinabove with reference to FIG. 1. Thegradation correction circuit 28 includes a quantization circuit 29, anarea discrimination filter 30, a lookup table (LUT) 31, and amultiplication circuit 12. The multiplication circuit 12 is similar tothat of the gradation correction circuit 8 of the television camera 1described hereinabove with reference to FIG. 1, and overlappingdescription of it is omitted here to avoid redundancy.

The quantization circuit 29 quantizes a pixel value of a brightnesssignal Y which forms an image pickup result VT to reduce the bit numberof the brightness signal Y and outputs the brightness signal Y of thereduced bit number. In particular, the quantization circuit 29 in thegradation correction circuit 28 executes, for each pixel value x(i, j),arithmetic processing of the following expression (20) with aquantization step Q set in advance to linearly quantize the pixel valuex(i, j) to obtain a pixel value x′(i, j) and outputs the pixel valuex′(i, j).x′(i,j)=int(x/Q+0.5)  (20)where int(a) is a function of discarding the fraction of a.

The area discrimination filter 30 is formed similarly to the areadiscrimination filter 10 described hereinabove with reference to FIG. 10except that it handles a signal of a different bit number.

The lookup table 31 forms a coefficient calculation circuit similar tothe coefficient calculation circuit 11 of the television camera 1 of thefirst embodiment described hereinabove with reference to FIG. 1 andproduces and outputs a correction coefficient g(i, j) using a lowfrequency component r(i, j) outputted from the area discriminationfilter 30 as an address. To this end, the lookup table 31 stores acorrection coefficient LUT(i) given by the following expression (21) asan ith address.LUT(i)=G(i×Q)  (21)

Where the gradation correction circuit 28 is employed, a pixel value isquantized in advance and necessary processing is performed with thequantized pixel value. Consequently, similar advantages to thoseachieved by the television camera 1 described hereinabove with referenceto FIG. 1 can be achieved. Further, since a correction coefficient isproduced using a lookup table, the processing of the entire apparatuscan be simplified as much. Furthermore, since a pixel value quantized inadvance is used, the construction of the area discrimination filter canbe simplified and the lookup table can be reduced in scale.

First Modification to the Second Embodiment

FIG. 19 shows a gradation correction circuit applied to a firstmodification to the television camera of the second embodiment describedabove with reference to FIG. 18. Referring to FIG. 19, the gradationcorrection circuit is generally denoted at 28A and applied in place ofthe gradation correction circuit 8A of the modified television camera 1described hereinabove with reference to FIG. 9 or the gradationcorrection circuit 28 described above with reference to FIG. 18. Thegradation correction circuit 28A includes a pair of arithmetic circuits13A and 13B, a quantization circuit 45, an area discrimination filter40, a lookup table (LUT) 41, and a pair of multiplication circuits 12Cand 12Y. The arithmetic circuits 13A and 13B and multiplication circuits12C and 12U are similar to those described hereinabove with reference toFIG. 9, and overlapping description of them is omitted here to avoidredundancy.

The quantization circuit 45 re-quantizes each brightness data y(i, j) toreduce the bit number of the brightness data y(i, j) and outputs thebrightness data y(i, j) of the reduced bit number. In particular, thequantization circuit 45 in the gradation correction circuit 28Aexecutes, for each pixel value y(i, j), arithmetic processing of thefollowing expression (22) with a quantization step Q set in advance tolinearly quantize the pixel value y(i, j) to obtain a pixel value yq(i,j) and outputs the pixel value yq(i, j).yq(i,j)=int{y(i,j)/Q+0.5}  (22)where int(a) is a function of discarding the fraction of a.

The area discrimination filter 40 is formed similarly to the areadiscrimination filter 30 described hereinabove with reference to FIG. 18except that it handles a signal of a different bit number.

The lookup table 41 forms a coefficient calculation circuit and outputsa correction coefficient g(i, j) using a low frequency component r(i, j)outputted from the area discrimination filter 40 as an address. To thisend, the lookup table 41 stores a correction coefficient LUT(i) given bythe following expression (23) as an ith address.LUT(i)=G(i×Q)  (23)

Where the gradation correction circuit 28A shown in FIG. 19 is employed,brightness data is quantized in advance and necessary processing isperformed with the quantized pixel value. Consequently, similaradvantages to those achieved by the television camera 1 describedhereinabove with reference to FIG. 1 can be achieved with a furthersimplified construction. Further, since a correction coefficient isproduced using a lookup table, the processing of the entire apparatuscan be simplified as much. Furthermore, since a pixel value quantized inadvance is used then, the construction of the area discrimination filtercan be simplified and the lookup table can be reduced in scale.

Third Embodiment

FIG. 20 is a block diagram showing a gradation correction circuitapplied to a television camera according to a third preferred embodimentof the present invention. The gradation correction circuit is generallydenoted at 38 and is adapted in place of the gradation correctioncircuit 28 described hereinabove with reference to FIG. 18. Thegradation correction circuit 38 includes a quantization circuit 29, anarea discrimination filter 30, a lookup table (LUT) 41, an interpolationcircuit 42, and a multiplication circuit 12. The quantization circuit29, area discrimination filter 30 and multiplication circuit 12 aresimilar to those of the gradation correction circuit 28 describedhereinabove with reference to FIG. 18, and overlapping description of itis omitted here to avoid redundancy. The gradation correction circuit 38thus includes the lookup table 41 and the interpolation circuit 42 inplace of the lookup table 31 of the gradation correction circuit 28.

The lookup table 41 in the gradation correction circuit 38 has a numberof addresses smaller than the number of levels which can be assumed bythe output value r(i, j) of the area discrimination filter 30, and isaccessed with a value of the output value r(i, j) whose predeterminedlower bits are omitted. When the lookup table 54 is accessed in thismanner, it outputs two addresses addr0(i, j) and addr1(i, j) representedby the following expressions (24) and two correction coefficients g0(i,j) and g1(i, j).addr0(i,j)=int{r(i,j)/Rmax×R′max}addr1(i,j)=addr0(i,j)+1  (24)where Rmax is a maximum value which can be assumed by the output valuex(i, j) of the area discrimination filter 30, and R′max is a maximumvalue which can be assumed by the address of the lookup table 41.

It is to be noted that the lookup table 41 produces the address addr0(i,j) by omitting lower bits of the output value r(i, j) of the areadiscrimination filter 30 and produces the address addr1(i, j) by addinga bit of the logic 1 to the lowest bit of the address addr0(i, j).

The interpolation circuit 42 executes interpolation arithmeticprocessing in accordance with the following expressions (25) using theaddresses addr0(i, j) and addr1(i, j) and the correction coefficientsg0(i, j) and g1(i, j) inputted thereto from the lookup table 41 andoutputs a result of the interpolation as a correction coefficient g(i,j).

$\begin{matrix}{{g( {i,j} )} = {{{\{ {{r^{\prime}( {i,j} )} - {{addr}\; 0( {i,j} )}} \}/\{ {{{addr}\; 1( {i,j} )} - {{addr}\; 0\;( {i,j} )}} \}} \times \{ {{g\; 1( {i,j} )} - {g\; 0( {i,j} )}} \}} + {g\; 0( {i,j} )}}} & \; \\{{r^{\prime}( {i,j} )} = {{{r( {i,j} )}/R}\;\max \times R^{\prime}\max}} & (18)\end{matrix}$

Where the gradation correction circuit 38 shown in FIG. 20 is employed,since interpolation arithmetic processing is performed to produce acorrection coefficient, a correction coefficient whose value exhibits asmooth variation can be produced using a lookup table of a comparativelysmall scale, and the gradation can be corrected with a higher degree ofaccuracy as much.

First Modification to the Third Embodiment

FIG. 21 shows a gradation correction circuit applied to a firstmodification to the television camera of the third embodiment describedabove with reference to FIG. 20. Referring to FIG. 21, the gradationcorrection circuit is generally denoted at 38A and applied in place ofthe gradation correction circuit 28A described hereinabove withreference to FIG. 19 or the gradation correction circuit 38 describedabove with reference to FIG. 20. The gradation correction circuit 38Aincludes a pair of arithmetic circuits 13A and 13B, a quantizationcircuit 45, an area discrimination filter 40, a lookup table 51, ainterpolation circuit 52, and a pair of multiplication circuits 12C and12Y. The arithmetic circuits 13A and 13B, quantization circuit 45, areadiscrimination filter 40, and multiplication circuit 12C and 12Y aresimilar to those described hereinabove with reference to FIG. 19, andoverlapping description of them is omitted here to avoid redundancy. Thegradation correction circuit 38 thus includes the lookup table 51 andthe interpolation circuit 52 in place of the lookup table 41 of thegradation circuit 28A.

The interpolation circuit 52 executes interpolation arithmeticprocessing in accordance with the expressions (24) given hereinaboveusing addresses addr0(i, j), addr1(i, j) and correction coefficientsg0(i, j), g1(i, j) inputted thereto from the lookup table 51 and outputsa result of the interpolation as a contrast correction coefficient g(i,j).

Where the gradation correction circuit 38A shown in FIG. 21 is employed,since interpolation arithmetic processing is performed to produce acorrection coefficient, a correction coefficient whose value exhibits asmooth variation can be produced using a lookup table of a comparativelysmall scale, and the gradation can be corrected with a higher degree ofaccuracy as much.

Fourth Embodiment

FIG. 22 is a block diagram showing a gradation correction circuitapplied to a television camera according to a fourth embodiment of thepresent invention. The gradation correction circuit is generally denotedat gradation correction circuit 48 and is adapted in place of thegradation correction circuit 8 described hereinabove with reference toFIG. 1. The gradation correction circuit 48 includes an areadiscrimination filter 50, a coefficient calculation circuit 11, and amultiplication circuit 12. The coefficient calculation circuit 11 andmultiplication circuit 12 are similar to those of the gradationcorrection circuit 8 described hereinabove with reference to FIG. 1, andoverlapping description of it is omitted here to avoid redundancy.

The area discrimination filter 50 in the gradation correction circuit 48includes a low-pass filter section 50A which discriminates an area towhich input image data belongs with different resolutions to obtain andoutput identification signals r0(i, 1), r1(i, j), r2(i, j), . . .rN−1(i, j), and a signal composition section 50B for producing anidentification result r(i, j) of a single composite signal based on theidentification results r0(i, 1), r1(i, j), r2(i, j), . . . rN−1(i, j)according to the different resolutions.

The low-pass filter section 50A is formed from low-pass filters (LPF)F0, F1, F2, . . . , FN−1 having different pass-band widths. A pixelvalue x(i, j) of a brightness signal Y produced from an image pickupresult VT is inputted to the low-pass filters F0, F1, F2, . . . , FN−1,and corresponding low frequency components are outputted asidentification results r0(i, j), r1(i, j), r2(i, j), . . . , rN−1(i, j)from the low-pass filters F0, F1, F2, . . . , FN−1, respectively.

The signal composition section 50B includes multiplication circuits M0,M1, M2, . . . , MN−1 which receive and weight the identification resultsr0(i, j), r1(i, j), r2(i, j), . . . , rN−1(i, j), and an additioncircuit 53 which adds the weighted identification results r0(i, j),r1(i, j), r2(i, j), . . . , rN−1(i, j) to produce a composite signal asan identification result r(i, j). The identification result r(i, j) thusobtained by the addition circuit 53 is outputted from the signalcomposition section 50B. It is to be noted that weighting coefficientsw0, w1, w2, . . . , wN−1 which are used by the multiplication circuitsM0, M1, M2, . . . , MN−1, respectively, are set in advance so that theymay satisfy the following relational expression (26):

$\begin{matrix}{{\sum\limits_{k = 0}^{N - 1}{wk}} = 1} & (26)\end{matrix}$

Consequently, in the gradation correction circuit 48 described above, aprofile provided by the image pickup result VT is not emphasizedabnormally according to the setting of the weighting coefficients w0,w1, w2, . . . , wN−1.

In particular, if the pixel value x(i, j) varies suddenly as seen inFIG. 23(A), then the signal level of the low frequency component r(i, j)varies so that such a sudden variation of the pixel value may bemoderated. When the signal level of the low frequency component r(i, j)of the pixel value x(i, j) is displaced to the higher level side withrespect to the inflection point of the characteristic describedhereinabove with reference to FIG. 6, if the contrast correctioncoefficient g(i, j) is produced merely based on an output signal of alow-pass filter as in the television camera 1 of FIG. 10, then the pixelvalue is amplified with an excessively high gain immediately before thepixel value x(i, j) varies suddenly, but immediately after the pixelvalue x(i, j) varies suddenly, the pixel value is amplified with anexcessively low gain. Consequently, an output value y(i, j) (FIG. 23(B))which provides an abnormally amplified profile is obtained.

In this instance, such abnormal emphasis of the profile as justdescribed can be reduced by correcting the pixel values with asubstantially uniform gain.

Consequently, where the gradation correction circuit 48 shown in FIG. 22is employed, since correction coefficients are produced from a pluralityof different low frequency components, abnormal emphasis of a profilecan be prevented effectively, and similar advantages to those achievedby the television camera 1 of the first embodiment described hereinabovewith reference to FIG. 1 can be achieved.

First Modification to the Fourth Embodiment

FIG. 24 shows a gradation correction circuit applied to a firstmodification to the television camera of the fourth embodiment shown inFIG. 22. Referring to FIG. 21, the gradation correction circuit isgenerally denoted at 48A and applied in place of the gradationcorrection circuit 8A described hereinabove with reference to FIG. 9 orthe gradation correction circuit 48 described above with reference toFIG. 22. The gradation correction circuit 48A includes a pair ofarithmetic circuits 13A and 13B, an area discrimination filter 30, acoefficient calculation circuit 11, and a pair of multiplicationcircuits 12C and 12Y. The arithmetic circuits 13A and 13B, coefficientcalculation circuit 11, and multiplication circuit 12C and 12Y aresimilar to those described hereinabove with reference to FIG. 9 whilethe area discrimination filter 30 is similar to the area discriminationfilter 50 described hereinabove with reference to FIG. 22, andoverlapping description of them is omitted here to avoid redundancy.

Fifth Embodiment

FIG. 25 is a block diagram showing a gradation correction circuitapplied to a television camera according to a fifth preferred embodimentof the present invention. The gradation correction circuit is generallydenoted at 58 and is adapted in place of the gradation correctioncircuit 8 described hereinabove with reference to FIG. 1. The gradationcorrection circuit 58 includes an area discrimination filter 60, acoefficient calculation circuit 61, and a multiplication circuit 12. Themultiplication circuit 12 is similar to that of the gradation correctioncircuit 8 described hereinabove with reference to FIG. 1, andoverlapping description of it is omitted here to avoid redundancy.

The area discrimination filter 60 in the gradation correction circuit 58outputs discrimination results r0(i, j), r1(i, j), r2(i, j), . . . ,rN−1(i, j) according to different resolutions. In particular, the areadiscrimination filter 60 is formed from low-pass filters (LPF) F0, F1,F2, . . . , FN−1 having different pass-band widths. A pixel value x(i,j) is inputted to the low-pass filters F0, F1, F2, . . . , FN−1, andcorresponding frequency components are outputted as identificationresults r0(i, j), r1(i, j), r2(i, j), . . . , rN−1(i, j) from thelow-pass filters F0, F1, F2, . . . , FN−1, respectively.

The coefficient calculation circuit 61 includes a coefficient productionsection 61A for producing, from the identification results r0(i, j),r1(i, j), r2(i, j), . . . , rN−1(i, j), corresponding correctioncoefficients g0(i, j), g1(i, j), g2(i, j), . . . , gN−1(i, j), and acoefficient composition section 61B for composing the correctioncoefficients g0(i, j), g1(i, j), g2(i, j), . . . , gN−1(i, j) to producea single correction coefficient g(i, j).

The coefficient production section 61A includes coefficient calculationsections L0, L1, L2, . . . , LN−1 for producing, from the identificationresults r0(i, j), r1(i, j), r2(i, j), . . . , rN−1(i, j), correspondingcorrection coefficients g0(i, j), g1(i, j), g2(i, j), . . . , gN−1(i, j)based on predetermined coefficient calculation functions Gk (k=0, 1, 2,. . . , N−1), respectively.

The coefficient composition section 61B includes multiplication circuitsM0, M1, M2, . . . , MN−1 which weight the correction coefficients g0(i,j), g1(i, j), g2(i, j), . . . , gN−1(i, j), and an addition circuit 63which adds results of the weighting by the multiplication circuits M0,M1, M2, . . . , MN−1 to produce and output a single correctioncoefficient g(i, j). It is to be noted that weighting coefficients w0,w1, w2, . . . , wN−1 used by the multiplication circuits M0, M1, M2, . .. , MN−1, respectively, are set in advance so that the relationalexpression (11) given hereinabove may be satisfied.

Where the gradation correction circuit 58 shown in FIG. 25 is employed,correction coefficients are produced from a plurality of different lowfrequency components and a single correction coefficient is producedfrom the produced correction coefficients, and consequently, similaradvantages to those achieved by the gradation correction circuit 48described hereinabove in the forth embodiment with reference to FIG. 22can be achieved.

First Modification to the Fifth Embodiment

FIG. 26 shows a gradation correction circuit applied to a firstmodification to the television camera of the fifth embodiment shown inFIG. 25. Referring to FIG. 26, the gradation correction circuit isgenerally denoted at 58A and applied in place of the gradationcorrection circuit 8A described hereinabove with reference to FIG. 9 orthe gradation correction circuit 58 described above with reference toFIG. 24. The gradation correction circuit 58A includes a pair ofarithmetic circuits 13A and 13B, an area discrimination filter 60, acoefficient calculation circuit 61, and a pair of multiplicationcircuits 12C and 12Y. The arithmetic circuits 13A and 13B, andmultiplication circuit 12C and 12Y are similar to those describedhereinabove with reference to FIG. 9 while the area discriminationfilter 60 and coefficient calculation circuit 61 are similar to thosedescribed hereinabove with reference to FIG. 25, and overlappingdescription of them is omitted here to avoid redundancy.

Sixth Embodiment

FIG. 27 is a block diagram showing a television camera according to asixth preferred embodiment of the present invention. The televisioncamera is generally denoted at 1 and includes a CCD solid-state imagepickup device 2, a timing generator (TG) 3, a pair of memories 4S and4N, an addition circuit 5, a level correction circuit 6, and a gradationcorrection circuit 68. The CCD solid-state image pickup device 2, timinggenerator 3, memories 4S and 4N, addition circuit 5, and levelcorrection circuit 6 are similar to those of the television camera 1 ofthe first embodiment described hereinabove with reference to FIG. 1.

The gradation correction circuit 68 includes an area discriminationfilter 15, a coefficient calculation circuit 11, and a multiplicationcircuit 12. The coefficient calculation circuit 11 and multiplicationcircuit 12 are similar to those of the television camera 1 of the firstembodiment described hereinabove with reference to FIG. 1.

The area discrimination filter 15 in the gradation correction circuit 68discriminates an area to which input image data belongs and outputs aresult of the discrimination. To this end, the area discriminationfilter 15 includes a pair of low-pass filters (LPF) 15A and 15B, aweighting coefficient production section 15C, a pair of multiplicationcircuits 15D and 15E, and an addition circuit 15F.

In particular, in the area discrimination filter 15, each pixel valuex(i, j) of an image pickup result VT is inputted to and band limited bythe low-pass filters 15A and 15B. In particular, the low-pass filters15A and 15B of the area discrimination filter 15 discriminate to whichaverage bright level area input image data belongs and output lowfrequency components r0(i, j) and r1(i, j) of results of thediscrimination, respectively. Further, in this instance, the low-passfilters 15A and 15B having different pass-band widths in the areadiscrimination filter 15 execute the respective processes simultaneouslyin parallel to each other to produce low frequency components r0(i, j)and r1(i, j) which are results of the discrimination with differentresolutions. It is to be noted that, in the television camera 1 of thepresent embodiment, in regard to the image pickup result VT inputted inthe order of raster scanning, the horizontal direction is indicated bythe subscript i while the vertical direction is indicated by thesubscript j as seen in FIG. 5.

The weighting coefficient production section 15C produces weightingcoefficients 1−w and w for the low frequency components r0(i, j) andr1(i, j) outputted from the low-pass filters 15A and 15B, respectively,by arithmetic processing in accordance with the following expressions(27) with reference to the low-pass filter 15A:

$\begin{matrix}{w = {{w\;\min\mspace{14mu}\ldots\mspace{14mu}{D( {r\; 0( {i,j} )} )}} < {D\;\min}}} \\{w = {{{\{ {{D( {r\; 0( {i,j} )} )} - {D\;\min}} \}/( {{D\;\max} - {D\;\min}} )} \times ( {{w\;\max} - {w\;\min}} )} + {w\;\min}}} \\{w = {{w\;\max\mspace{14mu}\ldots\mspace{14mu} D\;\min} \leq {D( {r\; 0( {i,j} )} )} < {D\;\max\mspace{14mu}\ldots\mspace{14mu} D\;\max} \leq {D( {r\; 0( {i,j} )} )}}}\end{matrix}$where Dmax and Dmin are constants for normalization, and wmax and wminare a maximum value and a minimum value of a value calculated as aweighting coefficient, respectively, and a value higher than 0 but lowerthan 1 is provided in advance to each of the values wmax and wmin. Thefunction D(L) is a function which depends upon the coefficientcalculation function G used by the succeeding coefficient calculationcircuit 11 and is defined by the following expression (28):

$\begin{matrix}{{D(L)} = {\frac{\mathbb{d}}{\mathbb{d}L}( {{G(L)} \times L} )}} & (28)\end{matrix}$

Consequently, the weighting coefficient production section 15C increasesthe value of the weighting coefficient w when the corresponding pixelvalue x(i, j) corresponds to a portion in which the gradient of a revelconversion function T(L) which is hereinafter described is small.

The multiplication circuits 15D and 15E weight the low frequencycomponents r0(i, j) and r1(i, j) with the weighting coefficients 1−w andw, respectively, and the succeeding addition circuit 15F adds results ofthe weighting by the multiplication circuits 15D and 15E to produce andoutput a single area discrimination result r(i, j).

Consequently, the area discrimination filter 15 executes weightedaddition processing of the following expression (29):r(i,j)=(1−w)×r0(i,j)+w×r1(i,j)  (29)so that, for an area which corresponds to a portion in which thegradient of the level conversion function T(L) is large, the ratio ofthe low frequency component r1(i, j) which has been band limited withthe higher resolution is increased to output a single areadiscrimination result r(i, j), but on the contrary, for another areawhich corresponds to a portion in which the gradient of the levelconversion function T(L) is small, the ratio of the low frequencycomponent r0(i, j) which has been band limited with the low resolutionis increased to output a single area discrimination result r(i, j).

Consequently, the area discrimination filter 15 produces adiscrimination result r(i, j) so that the spatial resolution may beswitched in response to the pixel value x(i, j) of the image pickupresult VT, that is, the spatial resolution of the discrimination resultr(i, j) may be lower with a portion in which the gradient of the levelconversion function T(L) which is an input/output characteristic of thecoefficient calculation circuit 11 which is hereinafter described issmaller.

Consequently, an area to which input image data of the image pickupresult VT is discriminated with the different resolutions and results ofthe discrimination are produced by the low-pass filters 15A and 15B ofthe area discrimination filter 15. More particularly, low frequencycomponents r0(i, j) and r1(i, j) which indicate average brightnesslevels which are average values with regard to a pixel value x(i, j) areextracted by the low-pass filters 15A and 15B for the differentfrequency bands, and consequently, fine structures in the image areremoved and areas in which pixel values are comparatively flat areextracted.

The two low frequency components r0(i, j) and r1(i, j) of the imagepickup result VT are composed into a single low frequency component r(i,j) by the weighted averaging circuit formed from the multiplicationcircuits 15D and 15E and the addition circuit 15F, and the low frequencycomponent r(i, j) is outputted as a discrimination result for each area.

Then, a contrast correction coefficient g(i, j) is produced inaccordance with the signal level of the low frequency component r(i, j)of the image pickup result VT by the succeeding coefficient calculationcircuit 11, and the pixel value of the result of image pickup iscorrected with the contrast correction coefficient g(i, j) by themultiplication circuit 12.

With the television camera 1 having the construction described abovewith reference to FIG. 27, since a correction coefficient is producedbased on a result of discrimination of an area to which each image databelongs and an image pickup result is corrected with the correctioncoefficient, while the relationship in magnitude among pixel values inthe same area is maintained because the same coefficient is used, pixelvalues which belong to different areas can be made nearer to each otherin accordance with the necessity, and in an extreme case, therelationship can be reversed. Consequently, the gradation can becorrected while preventing partial deterioration of the contrast.

In this instance, if the discrimination results according to thedifferent resolutions are composed such that a correction coefficientfor a comparatively low resolution is allocated to a value area formedfrom a level with which the gradient of the level conversion curve issmall whereas another correction coefficient for a comparatively highresolution is allocated to another area formed from a level with whichthe gradient of the level conversion curve is great, then a naturalcontrast can be assured also between adjacent areas, and the gradationcan be corrected further naturally.

First Modification to the Sixth Embodiment

FIG. 28 shows a gradation correction circuit applied to a firstmodification to the television camera 1 of the sixth embodiment shown inFIG. 27. Referring to FIG. 28, the gradation correction circuit isgenerally denoted at 68A and applied in place of the gradationcorrection circuit 68 described hereinabove with reference to FIG. 27.The gradation correction circuit 68A includes an area discriminationfilter 19, a coefficient calculation circuit 21, and a multiplicationcircuit 12. The multiplication circuit 12 is similar to that describedhereinabove with reference to FIG. 27, and overlapping description of itis omitted here to avoid redundancy.

The area discrimination filter 19 in the gradation correction circuit68A outputs discrimination results r0(i, j) and r1(i, j) obtained bydiscriminating an area to which a pixel value x(i, j) belongs withdifferent resolutions.

In particular, the area discrimination filter 19 includes a pair oflow-pass filters (LPF) 19A and 19B having different pass-band widths. Apixel value x(i, j) is provided to the low-pass filters 19A and 19B, andcorresponding low frequency components are outputted as discriminationresults r0(i, j) and r1(i, j) from the low-pass filters 19A and 19B,respectively.

The low-pass filters 19A and 19B are formed similarly to the low-passfilters 15A and 15B described hereinabove with reference to FIG. 27,respectively.

The coefficient calculation circuit 21 includes a pair of coefficientcalculation sections 21A and 21B, a weighting coefficient productionsection 21C, a pair of multiplication circuits 21D and 21E, and anaddition circuit 21F, and produces, from the discrimination resultsr0(i, j), r1(i, j), corresponding correction coefficients g0(i, j),g1(i, j) and composes the two correction coefficients g0(i, j), g1(i, j)to produce a single contrast correction coefficient g(i, j).

In particular, the coefficient calculation sections 21A and 21B in thecoefficient calculation circuit 21 produce correction coefficients g0(i,j), g1(i, j) from the discrimination results r0(i, j), r1(i, j) based onrespective predetermined coefficient calculation functions Gk (k=0, 1)and outputs the thus produced correction coefficients g0(i, j), g1(i,j).

The weighting coefficient production section 21C executes arithmeticprocessing similar to that executed with regard to the expression (4)given hereinabove with reference to a pixel value x(i, j) of the imagepickup result VT. Thus, the coefficient production circuit 21C reducesthe value of the weighting coefficient w when the pixel value x(i, j)corresponds to an area in which the gradient of the level conversionfunction is small.

The multiplication circuits 21D and 21E weight the correctioncoefficients g0(i, j), g1(i, j) with the weighting coefficients 1−w andw, respectively, and the succeeding addition circuit 21F adds results ofthe weighting by the multiplication circuits 21D and 21E to produce asingle correction coefficient g(i, j) and outputs the correctioncoefficient g(i, j).

Consequently, the coefficient calculation circuit 21 operates thecorrection coefficients g0(i, j), g1(i, j) in response to the pixelvalue x(i, j) to switch the spatial resolution of the correctioncoefficient g(i, j).

In particular, the coefficient calculation circuit 21 produces thecontrast correction coefficient g(i, j) so that the spatial resolutionof the correction coefficient g(i, j) may be lower with a portion inwhich the gradient of the level conversion function which is aninput/output characteristic of the multiplication circuit 12 is smaller.

More specifically, the coefficient calculation circuit 21 increases, foran area which corresponds to a portion in which the gradient of thelevel conversion function T(L) is large, the ratio of the correctioncoefficient g1(I, j) generated by the low frequency component r1(i, j)which has been band limited with the higher resolution to output asingle correction coefficient g(i, j), but increases, on the contrary,for another area which corresponds to a portion in which the gradient ofthe level conversion function T(L) is small, the ratio of the correctioncoefficient g0(i, j) generated by the low frequency component r0(i, j)which has been band limited with the low resolution to output a singlecorrection coefficient g(i, j).

Where the gradation correction circuit 68A having the constructiondescribed above with reference to FIG. 28 is employed, since correctioncoefficients according to different resolutions are produced andcomposed into a single correction coefficient and the gradation iscorrected with the correction coefficient, advantages similar to thoseachieved by the television camera according to the sixth embodiment ofthe present invention can be achieved.

Other Forms

It is to be noted that, while, in all of the embodiments describedabove, a correction coefficient is produced basically with acharacteristic described hereinabove with reference to FIG. 6, thepresent invention is not limited to this, and a correction coefficientmay be produced with any of various input/output characteristics. Forexample, a level conversion function may be used which provides such aninput/output characteristic wherein the output level decreasesintermediately as the input level increases as seen in FIG. 29.

In particular, according to a conventional technique, where such afunction as described above is used, since it is not a monotoneincreasing function, a false profile sometimes appears on an image of aresult of processing. However, where image data are divided intodifferent areas by means of low-pass filters and processing is performedfor the image data of the different areas as in the embodimentsdescribed hereinabove, such a large variation of a pixel value thatcauses reversal of a relationship in magnitude between pixel values canbe prevented within each neighboring area of a magnitude correspondingto the pass band of the corresponding filter. Consequently, appearanceof a false profile can be prevented effectively.

Further, while, in the embodiments described above, a coefficientcalculation function G is produced by arithmetic processing of theexpression (6) using the level conversion function T, the presentinvention is not limited to this, and the coefficient calculationfunction G may be set arbitrarily without using the level conversionfunction T.

Furthermore, while, in the embodiments described above, a gradation iscorrected by a gradation correction circuit and then the dynamic rangeis suppressed by a succeeding signal processing circuit, the presentinvention is not limited to this, and such processes may be executedcollectively in accordance with the setting of the level conversionfunction T and the corresponding coefficient calculation function G.

In particular, in the process of suppressing the dynamic range, it isrequired that the number of bits of a pixel value to be outputted besmaller than the number of bits of a pixel value inputted, and theprocesses described above can be executed collectively by setting themaximum value of the output level to a maximum value permitted to anoutput image in the level conversion function T and producing acoefficient calculation function G using the maximum value thus set.

Where the coefficient calculation function G is set arbitrarily withoutusing the level conversion function T, the coefficient calculationfunction G should be set so as to satisfy the following expressions(30):L×G(L)≦LOmax0≦L≦Lmax  (30)where L is the input pixel level, Lmax is the maximum value of the inputpixel level, and LOmax is the maximum value of the output pixel level.

Further, while, in the second to the sixth embodiments described above,a quantization circuit, a lookup table and an interpolation circuit areused, the present invention is not limited to this, and all or some of aquantization circuit, a lookup table and an interpolation circuit may beapplied if necessary to the apparatus other than the second to sixthembodiments.

Or conversely, a quantization circuit may be omitted if necessary fromthe second to sixth embodiments.

Furthermore, while, in the embodiments described above, a brightnesssignal is produced from a color signal and the gradation of the colorsignal is corrected with reference to the brightness signal, the presentinvention is not limited to this and can be applied widely to a casewherein an image pickup result (FIG. 3) wherein an amplitude modulatedcolor signal is superposed on a brightness signal outputted from asolid-state image pickup device of the single plate type is processedbased on, for example, such setting of a color filter as shown in FIG.2, another case wherein a video signal composed of a brightness signaland a color difference signal is processed, a further case wherein acomposite video signal wherein a chroma signal is superposed on abrightness signal is processed, and so forth.

It is to be noted that, for example, where an image pickup resultwherein an amplitude modulated color signal is superposed on abrightness signal is processed, the gradation can be corrected whilepreventing color noise effectively by setting the resolution of thecorrection coefficient lower than the modulation frequency of the colorsignal.

Where a video signal composed of a brightness signal and a colordifference signal is processed, the gradation of the video signal can becorrected by calculating a correction coefficient based on thebrightness signal and correcting the gradations of the brightness signaland the color difference signal with the correction coefficient.

Further, while, in the embodiments described above, an area to whicheach input image data belongs is discriminated with low-pass filters andlow frequency components outputted from the low-pass filters are used asa discrimination result, the present invention is not limited to this,and similar advantages to those of the embodiments described above canbe achieved also by dividing a processing object image into severalareas with various characteristic amounts using various processingmethods such as, for example, by grasping the similarity between a pixelselected arbitrarily from an image of a processing object andneighboring pixels around the pixel, expanding the area successivelyfrom the pixel to divide the processing object area into several areasand then using the characteristic amount as a discrimination result.

Furthermore, while, in the embodiments described above, the presentinvention is applied to a television camera, the present invention isnot limited to this and can be applied widely to various imageprocessing apparatus such as a television receiver, a video taperecorder and a printer.

As described above, according to the present invention, since an area towhich each input image data belongs is discriminated, for example, withreference to a low frequency component of a pixel value and the signallevel of the image data is corrected based on a result of thediscrimination, the gradation can be corrected while preventing partialdeterioration of the contrast effectively.

Further, since brightness data is demultiplexed from image data and acorrection coefficient is produced based on a result of discriminationof an area to which the brightness data belongs and then used forcorrection of the pixel value, the gradation can be corrected whilepreventing partial deterioration of the contrast effectively.

Furthermore, since a characteristic amount representative of acharacteristic of a predetermined range neighboring to each pixel isdetected and an area to which the input image data belongs isdiscriminated based on the characteristic amount and then a correctioncoefficient is produced based on a result of the discrimination and usedfor correction of the pixel value, where an output signal of, forexample, a solid-state image pickup device of the single plate type isselected as a processing object, the gradation can be corrected whilepreventing partial deterioration of the contrast effectively.

Besides, since, when a correction coefficient is produced based on adiscrimination result of an area to which input image data belongs and apixel value is corrected with the correction coefficient, the operationis switched such that the spatial resolution of the correspondingcorrection coefficient may be switched in response to the pixel value ofthe image data, the gradation can be corrected while preventing partialdeterioration of the contrast effectively, and in this instance, anatural contrast can be assured also between adjacent areas.

While preferred embodiments of the present invention have been describedusing specific terms, such description is for illustrative purposesonly, and it is to be understood that changes and variations may be madewithout departing from the spirit or scope of the following claims.

1. An image processing apparatus capable of correcting gradation ofimage data formed from a brightness signal and a color signalsequentially superposed on the brightness signal in a time divisionalrelationship, comprising: characteristic amount detection means forsuccessively detecting a characteristic amount indicative of apredetermined range neighboring to pixels of the image data; areadiscrimination means for discriminating areas to which the image databelong based on the characteristic amount and outputting discriminationresults; coefficient calculation means for outputting correctioncoefficients to be used for correcting the pixel values of the imagedata based on the discrimination results; and correction means forcorrecting the pixel values of the image data using the correctioncoefficients, wherein the coefficient calculation means sets thecorrection coefficients at a fixed value when a signal level of thediscrimination results is less than or equal to a prescribed value, andthe coefficient calculation means sets the correction coefficients at avalue less than the fixed value when the signal level of thediscrimination results is greater than the prescribed value.
 2. An imageprocessing apparatus according to claim 1, wherein said characteristicamount detection means successively detects a maximum value of the pixelvalues in the predetermined neighboring range as the characteristicamount.
 3. An image processing apparatus according to claim 1, whereinsaid characteristic amount detection means successively detects aminimum value of the pixel values in the predetermined neighboring rangeas the characteristic amount.
 4. An image processing apparatus accordingto claim 1, wherein said characteristic amount detection meanssuccessively detects a maximum value and a minimum value of the pixelvalues in the predetermined neighboring range and detects thecharacteristic amount based on the maximum value and the minimum value.5. An image processing apparatus according to claim 1, wherein saidcharacteristic amount detection means successively detects a maximumvalue and a minimum value of the pixel values in the predeterminedneighboring range and composes the maximum value and the minimum valuein response to an average value of the image data to detect thecharacteristic amount.
 6. An image processing apparatus according toclaim 1, wherein said area discrimination means includes a low-passfilter for extracting a low frequency component of the characteristicamount, and said coefficient calculation means produces the correctioncoefficients in response to the low frequency components received fromsaid low-pass filter.
 7. An image processing apparatus according toclaim 1, wherein said area discrimination means includes quantizationmeans for quantizing the characteristic amount, and a low-pass filterfor extracting a low frequency component from the characteristic amountquantized by said quantization means, and said coefficient calculationmeans produces the correction coefficients in response to the lowfrequency components received from said low-pass filter.
 8. An imageprocessing apparatus according to claim 1, wherein said areadiscrimination means includes a plurality of low-pass filters forindividually extracting low frequency components of the characteristicamount, and signal composition means for producing single compositesignals based on the low frequency components outputted from saidlow-pass filters, and said coefficient calculation means produces thecorrection coefficients based on the composite signals received fromsaid signal composition means.
 9. An image processing apparatusaccording to claim 8, wherein said signal composition means weightedaverages the low frequency components outputted from said low-passfilters to produce the composite signals.
 10. An image processingapparatus according to claim 8, wherein said signal composition meansweighted adds the low frequency components outputted from said low-passfilters with weighting coefficients set in advance to produce thecomposite signals.
 11. An image processing apparatus according to claim1, wherein said area discrimination means includes a plurality oflow-pass filters for individually extracting low frequency components ofthe characteristic amount, and said coefficient calculation meansincludes partial coefficient calculation means for producingcoefficients for correction from the low frequency components outputtedfrom said low-pass filters, and coefficient composition means forproducing the correction coefficients based on the coefficients forcorrection.
 12. An image processing apparatus according to claim 11,wherein said coefficient composition means weighted averages thecoefficients for correction to produce the correction coefficients. 13.An image processing apparatus according to claim 11, wherein saidcoefficient composition means weighted adds the coefficients forcorrection with weighting coefficients set in advance to produce thecorrection coefficients.
 14. An image processing apparatus according toclaim 1, wherein said correction means multiplies the pixel values ofthe image data by the correction coefficients to correct the pixelvalues of the image data.
 15. An image processing apparatus according toclaim 1, wherein the number of bits of the image data outputted fromsaid correction means is smaller than the number of bits of the imagedata inputted to said characteristic amount detection means.
 16. Animage processing method for correcting the gradation of image dataformed from a brightness signal and a color signal sequentiallysuperposed on the brightness signal in a time divisional relationship,comprising: using a hardware device to perform: successively detecting acharacteristic amount indicative of a predetermined range neighboring topixels of the image data; discriminating areas to which the image databelong based on the characteristic amount and outputting discriminationresults; setting correction coefficients at a fixed value when a signallevel of the discrimination results is less than or equal to aprescribed value; setting the correction coefficients at a value lessthan the fixed value when the signal level of the discrimination resultsis greater than the prescribed value; outputting the correctioncoefficients to be used for correcting the pixel values of the imagedata based on the discrimination results; and correcting the pixelvalues of the image data using the correction coefficients.
 17. An imageprocessing method according to claim 16, wherein the characteristicamount detection step successively detects a maximum value of the pixelvalues in the predetermined neighboring range as the characteristicamount.
 18. An image processing method according to claim 16, whereinthe characteristic amount detection step successively detects a minimumvalue of the pixel values in the predetermined neighboring range as thecharacteristic amount.
 19. An image processing method according to claim16, wherein the characteristic amount detection step successivelydetects a maximum value and a minimum value of the pixel values in thepredetermined neighboring range and detects the characteristic amountbased on the maximum value and the minimum value.
 20. An imageprocessing method according to claim 16, wherein the characteristicamount detection step successively detects a maximum value and a minimumvalue of the pixel values in the predetermined neighboring range andcomposes the maximum value and the minimum value in response to anaverage value of the image data to detect the characteristic amount. 21.An image processing method according to claim 16, wherein the areadiscrimination step extracts a low frequency component of thecharacteristic amount, and the coefficient calculation step produces thecorrection coefficients in response to the low frequency components. 22.An image processing method according to claim 16, wherein the areadiscrimination step quantizes the characteristic amount and extracts alow frequency component from the characteristic amount quantized by thequantization, and the coefficient calculation step produces thecorrection coefficients in response to the low frequency components. 23.An image processing method according to claim 16, wherein the areadiscrimination step includes a signal extraction step of extracting aplurality of low frequency components of the characteristic amount withdifferent frequency bands, and a signal composition step of producingsingle composite signals based on the low frequency components., and thecoefficient calculation step produces the correction coefficients basedon the composite signals.
 24. An image processing method according toclaim 23, wherein the signal composition step weighted averages the lowfrequency components to produce the composite signals.
 25. An imageprocessing method according to claim 23, wherein the signal compositionstep weighted adds the low frequency components with weightingcoefficients set in advance to produce the composite signals.
 26. Animage processing method according to claim 16, wherein the areadiscrimination step extracts a plurality of low frequency components ofthe characteristic amount with different frequency bands, and thecoefficient calculation step includes a partial coefficient calculationstep of producing coefficients for correction from the low frequencycomponents, and a coefficient composition step of producing thecorrection coefficients based on the coefficients for correction.
 27. Animage processing method according to claim 26, wherein the coefficientcomposition step weighted averages the coefficients for correction toproduce the correction coefficients.
 28. An image processing methodaccording to claim 26, wherein the coefficient composition step weightedadds the coefficients for correction with weighting coefficients set inadvance to produce the correction coefficients.
 29. An image processingmethod according to claim 16, wherein the correction step multiplies thepixel values of the image data by the correction coefficients to correctthe pixel values of the image data.
 30. An image processing methodaccording to claim 16, wherein the number of bits of the image dataoutputted from the correction step is smaller than the number of bits ofthe image data inputted.